Are these fusion drives in the new iMac really that bad? Everyone on here really has me apprehensive about buying an iMac with them. I really feel like it's my only option. I have a large photo/video/and music library which is around 1tb. I've tried moving my photos on to an external to make room on my existing iMac but it wouldn't move the library as it said some files couldn't be read. I'd really rather not us an external except for backups. I was planning on buying a 3tb fusion drive but it seems most people on here recommend getting an SSD and using an external.
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Fusion Drives
- Thread starter jw3571
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The 3tb fusion drive only has 128gb of ssd in it, which some people can easily surpass for files and programs which they use regularly. If you're just storing music, I wouldn't spend the money on pure SSD. I don't believe the majority of people need the SSD, they just see it as a "nice to have" option.
The main drawback is the small SSD on the 1TB option. It used to be 128GB like the 2TB/3TB, but Apple then shrunk it to 24GB to lower the retail price (though they have increased it to 32GB with the mid-2017 models - likely because APFS stores all the metadata on the SSD).
Since you are buying the 3TB model, you will have a 128GB SSD and that should honestly be sufficient to hold your most-recently-accessed applications and data so performance should generally be excellent.
Since you are buying the 3TB model, you will have a 128GB SSD and that should honestly be sufficient to hold your most-recently-accessed applications and data so performance should generally be excellent.
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I think there's a good number of folks that come here with either a bad experience, or just a poor attitude.
Consider that there is not currently an Apple repair program to repair/replace fusion drives, nor, as far as I have read, any general internal (unannounced) repair issues generally affected by fusion drives, then I think you can assume that most iMac users really don't have appreciable problems with fusion drives.
Apple has sold large numbers of iMacs sold with fusion drives, and those users that may post here --- don't come here often to report that they have no problems. Users with problems are those who might post here, and will be a pretty small percentage of total Mac owners.
And, the basic performance that you get with an SSD as the system boot drive is pretty good. I understand that you may have cost issues with a pure SSD system, but the compromise to get a decent amount of storage still makes the fusion drive a reasonable value, and one that you can expect to be reliable.
Consider that there is not currently an Apple repair program to repair/replace fusion drives, nor, as far as I have read, any general internal (unannounced) repair issues generally affected by fusion drives, then I think you can assume that most iMac users really don't have appreciable problems with fusion drives.
Apple has sold large numbers of iMacs sold with fusion drives, and those users that may post here --- don't come here often to report that they have no problems. Users with problems are those who might post here, and will be a pretty small percentage of total Mac owners.
And, the basic performance that you get with an SSD as the system boot drive is pretty good. I understand that you may have cost issues with a pure SSD system, but the compromise to get a decent amount of storage still makes the fusion drive a reasonable value, and one that you can expect to be reliable.
For the cost of the 3TB Fusion Drive option. You can instead get the 256GB SSD and add an external 8TB HDD. Plus you get complete control over what goes where. Instead of the 8TB external you can buy a 3TB for storage and 5TB for Time Machine. Either way you go. The SSD plus external gets you much more for the money.
On a side note. At least for the 27" models. Skip their RAM upgrade. For the price they charge for 16GB you can upgrade to 40GB. You could do 32GB on the 21" if you are brazen enough to open it up.
On a side note. At least for the 27" models. Skip their RAM upgrade. For the price they charge for 16GB you can upgrade to 40GB. You could do 32GB on the 21" if you are brazen enough to open it up.
What type of files are you talking about? iTunes and Photos?
I moved my iTunes media to the network. It's actually on my NAS. When I reboot it automatically mounts the NAS' iTunes directory.
Photos' library can be on an external drive but NOT on the NAS. The drive has to be HFS+ formatted.
I moved my iTunes media to the network. It's actually on my NAS. When I reboot it automatically mounts the NAS' iTunes directory.
Photos' library can be on an external drive but NOT on the NAS. The drive has to be HFS+ formatted.
OP asked:
"Are these fusion drives in the new iMac really that bad?"
Well, not that bad, IF you choose the right one.
You DON'T WANT the "1tb fusion" option, because the SSD portion of that one is only 24gb.
You MIGHT want the "2tb" or "3tb" (if they still offer that one) options. These have 128gb SSD portions.\
128gb is not "huge", but adequate enough for good performance. Folks who have the 2tb fusion configurations report that they are snappy performers.
My advice these days is:
UNLESS you HAVE TO HAVE a large amount of internally-based storage, it's probably a better choice to pick either the 256gb or 512gb internal SSD options (depending on whether or not you're will to pony up an additional $100/$300 at purchase time).
Either one is VERY fast, and will assure FAST performance over the entire life of the computer.
IF your storage needs are great, you should supplement the internal drive with an external USB3.1 drive.
That can be either platter-based or an SSD, again depending on how much you wish to spend.
"Are these fusion drives in the new iMac really that bad?"
Well, not that bad, IF you choose the right one.
You DON'T WANT the "1tb fusion" option, because the SSD portion of that one is only 24gb.
You MIGHT want the "2tb" or "3tb" (if they still offer that one) options. These have 128gb SSD portions.\
128gb is not "huge", but adequate enough for good performance. Folks who have the 2tb fusion configurations report that they are snappy performers.
My advice these days is:
UNLESS you HAVE TO HAVE a large amount of internally-based storage, it's probably a better choice to pick either the 256gb or 512gb internal SSD options (depending on whether or not you're will to pony up an additional $100/$300 at purchase time).
Either one is VERY fast, and will assure FAST performance over the entire life of the computer.
IF your storage needs are great, you should supplement the internal drive with an external USB3.1 drive.
That can be either platter-based or an SSD, again depending on how much you wish to spend.
You can, but you'll end up having a tiny SSD and clunky external enclosure and a plenty of manual management via symlinks, spin-up/down issues, noise, etc. You'll also get HDD speeds on anything on external drive. Sometimes having a control isn't the best option.
The best thing is that on FD you don't have to make that options what should go where. It is a task for software (LVM manager) to ensure that all your writes get cached and frequently used blocks stay on SSD. I want software to do this for me.
I am now running a fusion drive consisting of 512GB SSD and 4TB HDD and with roughly 2TB of media library on it (mostly Lightroom catalogue with RAWs and some videos). No chance I would break it down into two volumes...
A few months ago I bought a second hand Mac mini with a self made fusion drive. After a while the HDD died and my Mac suddenly didn't boot up anymore. Because of the fusion drive any data was lost, even on the working ssd. Since this experience I will never buy a FD again, plus FD is noticeably slower than stand alone ssd.
Thanks for the responses. One of my problems is I don't quite understand how to get my existing photos and videos over to an external. I tried copying my photo's library to an external but i got an error message saying some of the files couldn't be read. It also seems like a pain to have to have all my future photos go to an external, it seems a lot easier to have them all on an internal drive.
If this is your first time doing it, then yes it'll be weird, painful, uncomfortable, etc. for a while until you get used to it. Now that I've been using externals for close to 10 years, it's weird not to have things on the external drives. Wait until you start getting enough stuff that you have externals dedicated solely to a specific type of file, such as music, or for just iTunes in general lol. Or when you start having backups of your backups backup.
As far as I know, you just need to have the external drive formatted as HFS+.
Personally I have about 125 GB in Photos, so I'll keep it on my 1 TB internal SSD for now. Once it hits around three times that size, I'll move the entire thing to an external SSD.
I have two late 2013 iMacs, one is Flash-only (256GB Flash), the other with a 3TB Fusion Drive (128GB Flash). In day-to-day use, over the past 3+ years, there's no perceptible difference in their performance. There have have also been no reliability issues with my Fusion iMac. It just works.
In all the debates and discussions of Fusion, there have been few reports of bad experiences with Fusion setups. If anything would be a magnet for people who have had problems, those threads would be. Bottom line, though, is that you have to backup any computer - both internal and external storage - regardless of how its data is stored. I've seen plenty of people with all-SSD laptops who have needed to erase/reinstall their drives, so ditching the mechanical HDD isn't a reliability cure-all.
With Fusion, the average user (leaving out someone with fairly exotic requirements) will have a computer with at least 1TB of internal storage that performs at 80%-90% of the speed of a machine with all-Flash, at the price of a machine with 256GB of Flash. If someone says it's cheaper to buy a 256GB all-Flash Mac and a large external HDD, he ignores the difference in performance - without Fusion, everything on the external drive will operate at HDD speeds, always.
Their assumption is that data doesn't have to be fast. With the exception of watching streaming videos (and other read-once-only data), they're simply wrong. As soon as data is manipulated in any way (read/write activity), it will benefit by being in faster storage. Crunching numbers in a spreadsheet or working with databases? Editing documents or a graphics file? Real working efficiency doesn't come from having a Mac that boots up quickly, or loads an app quickly. Both of those things happen about once a day, if that. The rest of the workday, you're likely to be working with data. With Fusion, that data will be in faster Flash storage, automatically.
Around these forums there's a contingent that consistently treats Fusion with suspicion. Almost universally, they have not used it (some purchased a Fusion-equipped machine and immediately "broke" the Fusion into a dual-HD configuration). Either they simply don't believe it can work, or they're emotionally invested in their decision to do the "separate SSD for OS and apps/separate HDD (internal or external) for data 'thing'," or they believe that their needs are so special that Fusion can't possibly work for them, or they hate spinning HDDs so much that they can't embrace it. And many make statements about Fusion that prove they don't know how it really works.
Or they say, "All SSD is better." Well, of course all SSD is preferable, if cost is no object, or your storage needs will all fit neatly on, say, 256 GB (as it does in the case of my all-Flash iMac). What Fusion does beautifully is use an expensive resource (Flash storage) to accelerate the performance of a computer with large internal storage capacity, with zero management on the part of the user. No thought to, "Where should I put this?" No need to manually shuttle files from a slow HDD to fast SSD in order to run data at SSD speeds. With Fusion, the OS automatically moves data and code to and from the SSD as needed, the same way it moves data and code to and from RAM and to and from the CPU. No hocus-pocus, no voodoo.
Just as with RAM and the CPU, in Fusion the computer moves blocks of data and code to and from Flash (filesystem blocks are typically 4k), not entire files, not entire multi-file apps, and certainly not entire data libraries - it fetches the parts of a file that are actually needed. You can't estimate the amount of Flash that will be used by Fusion by looking at the size of the various folders or libraries on your HD, any more than you would try to have enough RAM to hold one of those folders.
When someone dedicates their expensive internal SSD to OS-and apps-only, a very substantial amount of that expensive storage is going to waste - few of us regularly use more than a small percentage of OS and app code. Generally, the OS-and-app-only drive will also have a fair amount of (expensive) free space that will never be used. Fusion, on the other hand, manages that expensive resource like RAM - it packs it full of stuff that is actively in use or was recently used, holding it in case it may be needed again, flushing out what hasn't been used in the longest while in favor of immediate need. So, while the 128GB of Flash that comes with 2TB and larger Fusion drives is very reassuring, if your daily work is done comfortably with 8GB of RAM, the 24GB of Flash in a 1TB Fusion drive will likely be enough, too.
As always, I've gone on way too long about this topic. My apologies. But since it's been so long since you've read my first paragraph, I'll repeat it, for emphasis:
I have two late 2013 iMacs, one is Flash-only (256GB Flash), the other with a 3TB Fusion Drive (128GB Flash). In day-to-day use, over the past 3+ years, there's no perceptible difference in their performance. There have also been no reliability issues with my Fusion iMac. It just works.
In all the debates and discussions of Fusion, there have been few reports of bad experiences with Fusion setups. If anything would be a magnet for people who have had problems, those threads would be. Bottom line, though, is that you have to backup any computer - both internal and external storage - regardless of how its data is stored. I've seen plenty of people with all-SSD laptops who have needed to erase/reinstall their drives, so ditching the mechanical HDD isn't a reliability cure-all.
With Fusion, the average user (leaving out someone with fairly exotic requirements) will have a computer with at least 1TB of internal storage that performs at 80%-90% of the speed of a machine with all-Flash, at the price of a machine with 256GB of Flash. If someone says it's cheaper to buy a 256GB all-Flash Mac and a large external HDD, he ignores the difference in performance - without Fusion, everything on the external drive will operate at HDD speeds, always.
Their assumption is that data doesn't have to be fast. With the exception of watching streaming videos (and other read-once-only data), they're simply wrong. As soon as data is manipulated in any way (read/write activity), it will benefit by being in faster storage. Crunching numbers in a spreadsheet or working with databases? Editing documents or a graphics file? Real working efficiency doesn't come from having a Mac that boots up quickly, or loads an app quickly. Both of those things happen about once a day, if that. The rest of the workday, you're likely to be working with data. With Fusion, that data will be in faster Flash storage, automatically.
Around these forums there's a contingent that consistently treats Fusion with suspicion. Almost universally, they have not used it (some purchased a Fusion-equipped machine and immediately "broke" the Fusion into a dual-HD configuration). Either they simply don't believe it can work, or they're emotionally invested in their decision to do the "separate SSD for OS and apps/separate HDD (internal or external) for data 'thing'," or they believe that their needs are so special that Fusion can't possibly work for them, or they hate spinning HDDs so much that they can't embrace it. And many make statements about Fusion that prove they don't know how it really works.
Or they say, "All SSD is better." Well, of course all SSD is preferable, if cost is no object, or your storage needs will all fit neatly on, say, 256 GB (as it does in the case of my all-Flash iMac). What Fusion does beautifully is use an expensive resource (Flash storage) to accelerate the performance of a computer with large internal storage capacity, with zero management on the part of the user. No thought to, "Where should I put this?" No need to manually shuttle files from a slow HDD to fast SSD in order to run data at SSD speeds. With Fusion, the OS automatically moves data and code to and from the SSD as needed, the same way it moves data and code to and from RAM and to and from the CPU. No hocus-pocus, no voodoo.
Just as with RAM and the CPU, in Fusion the computer moves blocks of data and code to and from Flash (filesystem blocks are typically 4k), not entire files, not entire multi-file apps, and certainly not entire data libraries - it fetches the parts of a file that are actually needed. You can't estimate the amount of Flash that will be used by Fusion by looking at the size of the various folders or libraries on your HD, any more than you would try to have enough RAM to hold one of those folders.
When someone dedicates their expensive internal SSD to OS-and apps-only, a very substantial amount of that expensive storage is going to waste - few of us regularly use more than a small percentage of OS and app code. Generally, the OS-and-app-only drive will also have a fair amount of (expensive) free space that will never be used. Fusion, on the other hand, manages that expensive resource like RAM - it packs it full of stuff that is actively in use or was recently used, holding it in case it may be needed again, flushing out what hasn't been used in the longest while in favor of immediate need. So, while the 128GB of Flash that comes with 2TB and larger Fusion drives is very reassuring, if your daily work is done comfortably with 8GB of RAM, the 24GB of Flash in a 1TB Fusion drive will likely be enough, too.
As always, I've gone on way too long about this topic. My apologies. But since it's been so long since you've read my first paragraph, I'll repeat it, for emphasis:
I have two late 2013 iMacs, one is Flash-only (256GB Flash), the other with a 3TB Fusion Drive (128GB Flash). In day-to-day use, over the past 3+ years, there's no perceptible difference in their performance. There have also been no reliability issues with my Fusion iMac. It just works.
This is an incredibly interesting, intelligently written and informative post that dispels some of the misinformation that is perpetuated on these forums.
I agree wholeheartedly with your sentiments.
I was playing around with the Fusion endowed 3.8 GHz 7600K iMac in the Apple Store on the weekend. It was noticeably slower in Photos than even the 15" MacBook Pro in that store, and very much slower than my 4.2 GHz 7700K iMac with SSD I have at home.
Not even in the same league. On the MBP and 7700K / SSD iMac, previews of the images were near instantaneous or at most 0.5 s to appear when scrolling through the database. On the 7600K / Fusion system, they took 0.5 to 5 seconds to appear.
IOW, it was a different league in UI feel. No it wasn't the end of the world, but it got pretty irritating on the Fusion system, I guess especially because I was used to a much faster UI.
I completely agree !
I have fusion drive Mac since 2013, and it works very well.
It's not the ultimate performance, but it's fast enough to be very pleasant to use.
My current iMac has the 1Tb FD (with 24Gb SSD) and it's really working well.
I recommend the larger fusion drive, but the small one is not as bas as people who have never used it think.
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Have you noticed that 80% of FD in stores are not well configured: look in diskutil, most of them are not "fused" and only the HDD storage is used.
And don't remember that FD "learns" the way you use your Mac to optimize the drive organization.
It becomes more and more efficient with time.
A Fusion Drive is like any other spinning disk - it will eventually fail. Then you have a major repair project, since iMac's are glued shut. AppleCare for $169 seems like a bargain if you buy a fusion drive.
To me, it's worth paying a bit extra for an SSD and peace of mind - the chances of it failing over the life of the iMac (say 5-7 years), is almost 0%. With a Fusion Drive, I'd guess there is a 10-50% chance (wave hands) you'll have to replace it over the life of the iMac, especially if the machine is left on most the time, with the disk spinning.
If there are some files you can't read, your disk has problems that should be fixed -before- you try and move everything to a new machine. Try cloning your iMac to an external drive, and boot off of that. If the cloning process doesn't "fix" things when copying, it should at least inform you what files are "bad", and you can remove them, or recover them from time machine. After that, you will be certain your new iMac will be as fast and as trouble free as possible.
To me, it's worth paying a bit extra for an SSD and peace of mind - the chances of it failing over the life of the iMac (say 5-7 years), is almost 0%. With a Fusion Drive, I'd guess there is a 10-50% chance (wave hands) you'll have to replace it over the life of the iMac, especially if the machine is left on most the time, with the disk spinning.
If there are some files you can't read, your disk has problems that should be fixed -before- you try and move everything to a new machine. Try cloning your iMac to an external drive, and boot off of that. If the cloning process doesn't "fix" things when copying, it should at least inform you what files are "bad", and you can remove them, or recover them from time machine. After that, you will be certain your new iMac will be as fast and as trouble free as possible.
I don't know how much time you spent playing with Photos on that demo machine, but I suspect you encountered the worst-case scenario - a library that may never have been opened. As I understand things, those demo machines have their disk images re-loaded at the start of every day - the Fusion SSD starts its day essentially empty (and depending on how they load the disk image, perhaps they are also breaking the Fusion drive)
. The first time data is read on a Fusion machine, it's coming to you at HDD speed, not SSD. It's the subsequent reads (and writes) that would occur at SSD speed.
I use Photos frequently; I have a 365GB library on the Fusion drive. When I scroll through the most recent years of images, scrolling and previews are smooth and lightning-fast - I visit those years frequently. If I go back to years I haven't accessed for a while, a screen full of previews will take about a second to load/resolve, but once loaded that first time, scrolling is on a par with the recent years.
This is expected behavior for Fusion, and the reason that benchmarking rates Fusion at 80-90% of SSD performance. That's an average, not an instantaneous number - once data is read into SSD, subsequent activity is going to be at 100% of SSD speed. That first read off of HDD is going to come at HDD speed, roughly 20% of SSD. That's the Fusion trade-off.
As I said previously, there's no doubt that all-Flash is preferable if cost is no object. No contest. Every read, every write, same blazing speed. But Fusion vs. working with that 365GB library off a spinning HDD with no SSD caching? Again, no contest.
For people contemplating an iMac with the entry-level spinning HDD, the $100 spent on Fusion will give a much greater performance boost, dollar for dollar, than anything else - more than bumping up from 8GB of RAM, more than upgrading a CPU.
Maybe $700 is just "a bit extra" to you, but that's the difference in cost between a 1TB Fusion iMac and the same machine with 1TB of Flash. That $700 would pay for two HDD replacements or one HDD replacement plus a bump-up from a 21.5" Retina with 1TB Fusion to a 27" Retina with 1TB Fusion.
I can't deny that the whole HDD failure thing is real - a spinning drive is much more likely to fail than a bank of Flash. If constant up-time or maximum performance is a significant concern, you bet I'd recommend Flash. Yet it's still a gamble as to when a spinner will fail. My early 2008 iMac is in constant-on usage, and still has its original HDD. It's also not common to have an HDD failure within the first three years (the life of an AppleCare plan). In a case where budget is important and usage is relatively non-demanding, I find 1TB or more of Flash a hard position to advocate.
You can also have a faulty SSD.
It takes 15mn to remove the screen and replace an HDD, it's not impossible.
In all cases, SSD, HDD, FD, you NEED to backup, so it's not a real problem for me.
It takes 15mn to remove the screen and replace an HDD, it's not impossible.
In all cases, SSD, HDD, FD, you NEED to backup, so it's not a real problem for me.
I had a lot of time to kill as I was waiting for an estimate on an iPhone mute button repair. It's $390 CAD dammit (for a replacement)!
I loaded up Photos about 10 times to test this. Each time the result was the same. Image previews were a lot slower on the Fusion drive. Again, not the end of the world because most people can wait a few seconds, but if you're used to SSD, it gets real old fast.
However, it should be noted that Fusion drives Apple sells now are not all the same. If you get a 2 TB Fusion drive, you get a 128 GB SSD. However, if you get a 1 TB Fusion Drive, the SSD is only 24 GB. 24! I didn't check the drive size, or if the drive was actually fused though. However, if it's a standard build, then it would have been a 2 TB Fusion drive, but it was a good point about the Fusion. I'd have to check that.
---
BTW, for the Canadian education store, the price premium of the 1 TB SSD is $648 CAD over than the standard 2 TB Fusion drive for the 3.8 GHz 7600K model. That ain't chicken feed, but is nowhere near as bad as it used to be. CAD$648 is US$490. In the US though for non-edu pricing, the same upgrade is US$600.
So I may be biased because I'm paying Canadian edu pricing for the upgrade, which is a lot cheaper than the $600 US non-edu pricing. BTW, US edu pricing is US$540, which is better, but it is still significantly more than Canadian edu pricing.
The larger internal SSD's options are expensive. The Fusion Drives are perfectly fine for most people. It's also a lot easier to deal with just one, large Fusion drive and an external TM disk.
The other thing to consider, is by using a more affordable 256 or 512G SSD, the OP has to use an external drive for data. And probably yet another external time machine - that's 3 drives. So, over the base model, assuming $200 for one external drive, and $300 for two of them:
1TB Fusion + 1 external TM, $0 + $200 = $200
2TB Fusion + 1 external TM, $200 + $200 = $400
256 SSD + 2 external (data and TM) = $100 + $300 = $400
512 SSD + 2 external (data and TM) = $300 + $300 = $600
1TB SSD + 1 external TM, $700 + $200 = $900
That's quite a range. If you ever use more than 1TB SSD internally, over the life of the iMac, you are probably going to have to use 2 external drives, so it's back to that.
Of course, many people never backup, at all. Or backup to the cloud..
What's frustrating, is that an iMac could easily be configured by Apple to have 1T SSD, plus any drive that fits in a 2.5" or 3.5" HDD bay. HDD's go up to 8-10T these days. But they don't offer it. Also, be aware if you buy an SSD only in an iMac, you don't get the cable to the 3.5" SATA drive bay. So even if you break open the iMac, you still can't add a drive in the 3.5" bay, without getting the cable from Apple (good luck with that..)
A lot of opinions here. I've had Fusion Drive iMacs since 2012 but for the 2017 model went pure SSD. Some observations:
- Speed. Pure SSD is considerably faster.
- Noise. Pure SSD is far quieter and has been a nice change.
- Reliability. When I installed Boot Camp on my 2015 iMac something went wrong and the Fusion Drive was destroyed. Apple doesn't make a utility that will easily re-create it. The only option is to spend a lot of time in Recovery Mode in the Terminal. It isn't fun at all. If Apple made it easier to repair a bad Fusion Drive I would discount the reliability arguments, but with how it is, I would avoid it if budget allows.
My digital media eventually has filled up the internal storage of every iMac I've owned, then I'm forced to slower external HDDs. However, with the new iMac I think I've found an upgrade path that won't require opening and unsealing the screen.
I'm going to start with an internal 512GB SSD and split it into two main partitions and install macOS into the first. Then the second internal 256GB SSD partition will be combined with an external HDD to create a fusion drive. Once this ext/int fusion drive fills up I tear down the fusion drive, replace the HDD with a bigger one, rebuild the new fusion drive, and restore my data from backups.
I'm going to start with an internal 512GB SSD and split it into two main partitions and install macOS into the first. Then the second internal 256GB SSD partition will be combined with an external HDD to create a fusion drive. Once this ext/int fusion drive fills up I tear down the fusion drive, replace the HDD with a bigger one, rebuild the new fusion drive, and restore my data from backups.
Is it a viable option to run an external SSD with MacOS on it, and effectively use the Fusion Drive as a secondary drive (i.e. without the OS and core apps on it)? Or is an external SSD going to run slower than an internal one?
I ask because I already have an external with MacOS on it, but if that's going to run noticeably slower than an internal SSD then I'll just bite the bullet and get a new iMac with a 512GB SSD and copy it over. But if my external SSD runs the same speed as an internal then I could get a Fusion Drive and make it function as a de facto 'external'.
I ask because I already have an external with MacOS on it, but if that's going to run noticeably slower than an internal SSD then I'll just bite the bullet and get a new iMac with a 512GB SSD and copy it over. But if my external SSD runs the same speed as an internal then I could get a Fusion Drive and make it function as a de facto 'external'.
I did this with my last machine, a Late 2013 iMac but even on that machine the internal SSD component of the Fusion Drive was about twice as fast. With the 2017 iMac it is more like four times the speed.
Go for the 512GB SSD. I did and now I use the external SSD for BootCamp Windows. You won't regret it.