Split 3TB Fusion Drive on iMac 5K and LOVE it!

tejota1911

macrumors 6502
Original poster
Nov 10, 2006
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I decided to split the Fusion Drive back into separate 128GB SSD and 3TB HDD drives on my iMac 5K, and I much prefer it to the FD setup. I run the SSD as my Boot Drive and the 3TB HDD for storage. I realize this won't be ideal for everybody, but if your considering doing this, go for it. It is very easy to do. I like having control over what I run off the SSD and what I store on the HDD. The SSD in the iMac 5K is PCIe based and very fast. Blackmagic disk speed tests are coming back around 320MB/s Writes, and 700MB/s Reads. I installed the OS and all programs on the SSD, and only run my iTunes directory from the storage drive(I have about 1TB of movies). Works great for me. Just thought I'd share in case someone out there was on the fence about doing this.
 

gnasher729

macrumors P6
Nov 25, 2005
16,510
3,102
After installing a home-made Fusion drive on my MacBook (exactly the opposite of what you did), all the album art started appearing very fast - even though my iTunes library comes nowhere close to fitting on the SSD drive. The reason is that Fusion is clever and actually has moved the album art of my music, books etc onto the SSD portion, and left the majority of data on the hard drive. You don't have that control with a split drive.

Nobody will (reasonably) claim that splitting a Fusion drive cannot be done or is very difficult. And if you prefer it that way, go for it. But I really wouldn't recommend it to anyone.
 

tejota1911

macrumors 6502
Original poster
Nov 10, 2006
283
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My album art loads very fast off the 3TB HDD as well. I keep iTunes open most of the time, since it is feeding my 3 AppleTVs, which I'm sure helps. Your MacBook HDD is most likely a 5400rpm 2.5" drive. Thats going to be significantly slower than the 3TB 7200rpm 3.5" drive on the iMac.

The problem with the FD is that Apple uses a 128GB SSD. If you have more than 128GB of data, the FD only keeps 4GB free on the SSD. If you deal with files larger than 4GB, it will constantly have to shuffle files around. In my case, I have 70GB free on my boot SSD, leaving roughly 50GB available for use.

I'm glad you like your home-brew Fusion Drive, but I WOULD recommend splitting the FD for those who would prefer to self manage their drives.
 

Truefan31

macrumors 68040
Aug 25, 2012
3,420
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Yeah I'd think if u got a 3 tb fusion drive they'd give more than 128 ssd. Maybe like at least 512
 

yjchua95

macrumors 604
Apr 23, 2011
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GVA, KUL, MEL (current), ZQN
I decided to split the Fusion Drive back into separate 128GB SSD and 3TB HDD drives on my iMac 5K, and I much prefer it to the FD setup. I run the SSD as my Boot Drive and the 3TB HDD for storage. I realize this won't be ideal for everybody, but if your considering doing this, go for it. It is very easy to do. I like having control over what I run off the SSD and what I store on the HDD. The SSD in the iMac 5K is PCIe based and very fast. Blackmagic disk speed tests are coming back around 320MB/s Writes, and 700MB/s Reads. I installed the OS and all programs on the SSD, and only run my iTunes directory from the storage drive(I have about 1TB of movies). Works great for me. Just thought I'd share in case someone out there was on the fence about doing this.
I went the pure SSD route instead and got 700-720MB/s writes and 730-750MB/s reads. More reliable and faster too.
 

ZipZap

macrumors 603
Dec 14, 2007
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Well if you think about it, giving you more than 128GB defeats the reason for Fusion all together. At that point you might as well use the SSD for boot and the HD for the rest.

OP, can you really get Yosemite to fit in 128GB comfortably?
 

torana355

macrumors 68040
Dec 8, 2009
3,391
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Sydney, Australia
My 2012 iMac has the 3tb fusion drive which i split, i then partitioned the 3TB HDD then recreated a fusion drive with one of the partitions. So i now have a 628GB fusion drive and a 2.5TB storage drive. This way i can keep files that i never want to go on the SSD separate. That being said i wont be buying another computer with a HDD, SSD all the way from here on in.
 

ZipZap

macrumors 603
Dec 14, 2007
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My 2012 iMac has the 3tb fusion drive which i split, i then partitioned the 3TB HDD then recreated a fusion drive with one of the partitions. So i now have a 628GB fusion drive and a 2.5TB storage drive. This way i can keep files that i never want to go on the SSD separate. That being said i wont be buying another computer with a HDD, SSD all the way from here on in.
That's not splitting the drive as you posted. But I do understand what you did. Do you really think you gained anything in this new configuration? I get pure SSD OS as I use to have a 512GB rMBP and all SSD is ideal. Its just not ideal at the Apple price points to get it.
 

torana355

macrumors 68040
Dec 8, 2009
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Sydney, Australia
That's not splitting the drive as you posted. But I do understand what you did. Do you really think you gained anything in this new configuration? I get pure SSD OS as I use to have a 512GB rMBP and all SSD is ideal. Its just not ideal at the Apple price points to get it.
Well what i gained is not having any of my video files touching the SSD, I understand the fusion drive is suppose to do this automatically but it is not perfect, my way eliminates any chance of Videos taking up precious SSD space. I also did this so i could install Windows on a partition of the storage drive before it was officially supported. (only the 1tb fusion drive was supported on release)
 

ZipZap

macrumors 603
Dec 14, 2007
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I honestly do not think that's how fusion works. The OS keeps if loaded with OS and most needed files.
 

tejota1911

macrumors 6502
Original poster
Nov 10, 2006
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OP, can you really get Yosemite to fit in 128GB comfortably?
Yosemite takes up about 8GB. So, yes I can get it to comfortably fit on a 128GB SSD. The base 13" rMBP only comes with 128GB of space total. With my setup I get that, plus 3TB of storage. Right now with all my programs, photos, etc. I'm using about 70GB of the SSD. That being said, I wouldn't mind if the SSD was larger.
 

ZipZap

macrumors 603
Dec 14, 2007
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Yosemite takes up about 8GB. So, yes I can get it to comfortably fit on a 128GB SSD. The base 13" rMBP only comes with 128GB of space total. With my setup I get that, plus 3TB of storage. Right now with all my programs, photos, etc. I'm using about 70GB of the SSD. That being said, I wouldn't mind if the SSD was larger.
I am positive that on a a brand new machine with nothing added, the OS uses much more that 8GB. The installer can fit on an 8GB thumb drive, but installed Yosemite is bigger.

So now that you are sure there a no videos, is it fair to say that approx 50GB of the SSD sits idle all the time? And the iMac has a 5400 rpm drive...so I can't imagine you aren't feeling the impact from that being uncached. In the end did you not say you recreated a 500GB fusion drive?
 

tejota1911

macrumors 6502
Original poster
Nov 10, 2006
283
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I am positive that on a a brand new machine with nothing added, the OS uses much more that 8GB. The installer can fit on an 8GB thumb drive, but installed Yosemite is bigger.

So now that you are sure there a no videos, is it fair to say that approx 50GB of the SSD sits idle all the time? And the iMac has a 5400 rpm drive...so I can't imagine you aren't feeling the impact from that being uncached. In the end did you not say you recreated a 500GB fusion drive?

I think you are confusing me and torana355, but yes I have 50GB free for use on the SSD. That's after the entire operating system, all my programs, and photos have been installed on it. I only run my iTunes directory from the 3TB HDD, which is 7200rpm not 5400rpm. Everything else runs solely from the SSD. The computer runs very well like this. For my uses, it runs BETTER like this than with the Fusion Drive enabled, hence me posting this thread. I am not running a fusion drive at all. Keep in mind, this is a 4.0GHz i7 Quad, 24GB RAM computer. Loading thumbnails in iTunes is not a problem. I am not feeling any impact from not caching my storage drive. YMMV.
 
Last edited:

Samuelsan2001

macrumors 604
Oct 24, 2013
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I am positive that on a a brand new machine with nothing added, the OS uses much more that 8GB. The installer can fit on an 8GB thumb drive, but installed Yosemite is bigger.

So now that you are sure there a no videos, is it fair to say that approx 50GB of the SSD sits idle all the time? And the iMac has a 5400 rpm drive...so I can't imagine you aren't feeling the impact from that being uncached. In the end did you not say you recreated a 500GB fusion drive?
Nope it only uses 8GB as anyone who has bought a new mac will tell you by the amount of disk space still available on the machine when new. A new rMBP 128GB has 120GB left available when it arrives...
 
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marzer

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Nov 14, 2009
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OP, can you really get Yosemite to fit in 128GB comfortably?
OS X doesn't even come close to needing that much space. Check the size of your own folders: /System + /Library + /Applications. You'll be surprised how small your executable (os + programs) storage actually is. I'm using 95GB total for program storage and that's with several large 3D games loaded, MS office, iWorks, lots of other third party programs and utilities. If I deduct the size of the games, my os + programs only take up 27GB.