Splitting Fusion Drive on 2017 iMac running Mojave

Discussion in 'iMac' started by rc705, May 5, 2019.

  1. rc705, May 5, 2019
    Last edited: May 5, 2019

    rc705 macrumors newbie

    Apr 13, 2019
    Hi everyone! Just purchased a used 2017 iMac 27" running Mojave. It came with a 3TB fusion drive which I want to split, then install a new SSD drive, while still using the blade to run the basic OS on.... but i'm having TROUBLE!

    I've followed some online guides where it shows using Terminal (diskutil cs list) to split them, but it seems to only work if the iMac was running High Sierra. Using Mojave doesnt detect the fusion CORE STORAGE system. I've read three (un-fact checked) things:

    1/ I can just simply install my new SSD and it'll automatically split the drives into two upon reboot.
    2/ I'll need to install MOJAVE on a separate flash then use Terminal to split them up, although, i can't find exact directions on how to do so.
    3/ Downgrade my system to High Sierra and it should work using Terminal

    Has anyone successfully split a Fusion Drive on an iMac with Mojave installed? If so, how'd you do it?

    Any advice would be greatly appreciated!! THANK YOU!!!
  2. Benz63amg macrumors 68030


    Oct 17, 2010
    Whats wrong with the 3TB Fusion drive that you want to install a new SSD?
  3. jerwin, May 5, 2019
    Last edited: May 5, 2019

    jerwin macrumors 68020

    Jun 13, 2015
    The 3tb portion uses spinning rust. One side effect of this is that bootcamp installs can be really slow, and windows doesn't like external drives.
  4. rc705 thread starter macrumors newbie

    Apr 13, 2019
    SSD is faster, more reliable and quieter.
  5. Benz63amg, May 5, 2019
    Last edited: May 5, 2019

    Benz63amg macrumors 68030


    Oct 17, 2010
    Spinning Rust? What a silly statement to make. So you think you know better than Apple? Every single iMac configuration stocked at Apple Stores nationwide in the US includes the Fusion Drive and Apple doesn't stock any iMac configuration with an SSD Only drive built in. Do you seriously think Apple would have opted to do this if the Fusion Drive was bad and "Spinning Rust" as you claim. Your post is an absolute joke claiming the Fusion Drive is "Spinning Rust".
  6. casperes1996 macrumors 68040


    Jan 26, 2014
    Horsens, Denmark
    Howdy, matey! Now I'd just like to add that you'd also have the option to incorporate the SSD into the Fusion setup. Any drive combination can be fused together, doesn't matter if it's 2 or 3 or 5 drives. You assign priorities based on speed and the OS handles the rest.

    Anyways, regarding splitting.

    If a Mac runs APFS (Apple File System), it won't use Core Storage to create the Fusion Drive like with HFS+. The short of it is that it's no longer necessary. Core Storage was created as a shim ontop of HFS+ to allow extra functionality, like Fusion Drives. This functionality can now be handled by the base file system itself with APFS.

    So instead of the diskutil cs commands
    we'll now be working with diskutil apfs.

    If you start by writing diskutil list apfs
    you'll get a list of apfs drives, containers and volumes.

    On each drive of the Fusion setup, there'll be a volume for the APFS container.
    The container is the result of the Fusion, and it'll say in the output of the command (Fusion) next to it.
    The two drives will have the names (Main) and (Secondary) where Main is the SSD.

    The key to separating the Fusion Drive under APFS, is to destroy the container.

    The following command exists
    deleteContainer (Delete an APFS Container and reformat disks to HFS)
    (prefix the command with diskutil apfs)

    As it mentions, it'll reformat the disks as hfs+ drives. Mojave won't install on HFS+ by default, so you'll have to format the drives individually to APFS afterwards.

    You do not need to boot from a separate installation of Mojave necessarily, but it is one way of doing it. Since splitting the Fusion means formatting the Mojave installation and the recovery drive however, you will need to do this from a non-active boot. I.e. Internet Recovery or a USB booter with the installer or an active system.
    I have not tested any of this in practice, but I know how the system works, and this should work, but I obviously give no guarantees, so you can't sue me for it ;)

    Let me know if you need any help.
  7. jerwin, May 5, 2019
    Last edited: May 5, 2019

    jerwin macrumors 68020

    Jun 13, 2015
    --Linus Torvalds (in 2012)

    The fact that SSDs are a Build to Order option really bothers me.
  8. casperes1996 macrumors 68040


    Jan 26, 2014
    Horsens, Denmark

    Linus is notoriously hyperbolic.

    And he's wrong in this case. Factually just wrong. SSDs cannot survive long term cold storage the way HDDs can. If you don't give an SSD any power at all for a year (typically a year on consumer drives, upwards of 3 on an enterprise) it'll have a high risk of having forgotten data. The cells just can't keep a charge for that long without getting a kick once in a while.
    SSDs are great for personal use, but that doesn't mean they don't still have their markets. Same way that magnetic tape storage is still widely used; Just not so much in the eye of the public.

    And Fusion Drives are actually quite great at what they do. Plus in some cases they can act like a striped RAID set, but with a disproportional distribution, actually speeding up certrain work traces even more than a single drive, SSD or otherwise, could've achieved.

    And the concept of Fusion extends beyond HDD/SSD combinations. It can be a fast and a slow SSD or combining Optane with regular NAND.

    Oh yeah, and show me a single SSD that holds 16TB like the latest Toshibe HDDs... The cold storage king is still a magnetic platter drive.

    Proper big data centres have been using Fusion-like caching techniques for ages with a proportionally small SSD set and a humongous HDD backend and they'll continue to do so going forward, because whilst you can get bigger SSDs for less now than ever before (though the big NAND producers are cutting production down to get supply down relative to demand again), you still can't compete with the capacity density and scaled cost of mechanical drives. And when a Fusion-like setup can get you so similar performance, the Fusion-like setup is preferred for massive amount of data that still need speed.
  9. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    If the Fusion drive is formatted APFS, there's nothing simple about this. It takes a combination of Disk Utility and Terminal and usually a few hours. Have a good backup because it will wipe the data on the drive.

    I do not like FDs but you have one. If everything is running nice and fast, why mess with it?

    High Sierra does not support APFS on FDs and HDDs; Mojave does.

    Which has nothing to do with anything the OP wants unless the goal is to install a 16TB HDD inside this 2017. Last I checked, 2017s have a 2.5" form.
  10. CoastalOR macrumors 68020


    Jan 19, 2015
    Oregon, USA
    It is unclear if you will keep the flash and SSD split or if you plan to fuse them.

    My suggestions:
    1. Make a good backup.
    2. Create a Mojave USB installer. Test it by booting from it.
    3. As you noted removing the the HDD of the FD will split the FD. Install the new SSD.
    4. Boot from the Mojave USB installer. Install Mojave to the flash drive. Use the backup to migrate data and settings to the Mojave boot flash drive.
    5. Use Disk Utility to format the new internal SSD if not already done before installation.
    --- Post Merged, May 5, 2019 ---
    Of course it is APFS. See the second sentence of the first post, "Just purchased a used 2017 iMac 27" running Mojave."
    Good advice.
    Already answered. See the OP's reply in post #4 for the reason that matters to the OP.
  11. rc705, May 5, 2019
    Last edited: May 5, 2019

    rc705 thread starter macrumors newbie

    Apr 13, 2019
    I'll be keeping them split (OS on the blade and everything else on the SSD). And I'd like to do it ASAP before I start doing anything with the computer. Losing data at this point is N/A since it only has the OS (Mojave). I just want to make sure I don't ruin the BLADE drive. I don't have any plans on what I'll do with the 3TB spinning drive afterwards.

    Im going to try installing Mojave on a flash thumb drive and see if I can boot from it. Then I'll have to find out how to Delete an APFS Container and reformat disks to HFS. Again, I can't find exact directions on how to do this, but lots of ideas on how it could be done.

    Thanks for the advice thus far everyone!!
    --- Post Merged, May 5, 2019 ---
    Ok, but HOW do I do this?
    Do I:
    1. boot from flash drive from thumb drive
    2. open terminal
    3. type in "deleteContainer" hit return
    4. then once down use disk utility to reformat the drives? (does disk utility give that option to format drives either in APFS or HFS?)
    5. then run a recovery and install Mojave onto the new separated BLADE drive

    it doesn't sound hard, but i just dont have exact details. lots of theories floating around, but haven't found step-by-step details anywhere.... which isn't reassuring.

    Thank you for all the insightful info! Much appreciated, sir!
  12. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    You looked at it through your long distance x-ray specs?

    There is No reason to assume that so I didn’t. It was purchased used, probably shipped with High Sierra which would not be APFS on that machine. Easy to defeat it on the upgrade. I wouldn’t but too many others would.
  13. rc705 thread starter macrumors newbie

    Apr 13, 2019
    it shipped with Mojave.
  14. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    Actually, it wasn’t answered. The OP claims an SSD is faster. Without knowing how it’s used, you have no way of knowing if it’s right. Neither do I which is why I gave the answer I did.
  15. rc705 thread starter macrumors newbie

    Apr 13, 2019
    The spinning drive is outdated tech and I want to keep this computer for at least 5 years. I thought it'd be easier to split the drives and then add in an SSD but this Mojave factor is throwing a monkey wrench into my plans. I'm currently using a 2010 iMac with an SSD I installed so I don't want to go backwards with harddrive tech on the 2017 iMac. And I'm on a budget or else I would have bought a brand new computer with an SSD installed already.
  16. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    Depends on the month. If it shipped with Mojave then my original answer is correct. You can’t just run a simple Terminal command and have it split.

    Apple doesn’t want you to just be able to do it. This doesn’t mean it can’t be done. You have a few choices but the easiest is to

    a) Make a bootable Mojave USB stick. I gave complete instructions in a recent post and will paste in the link.

    b) Replace the SSD–this gives you your biggest bang for the buck. A 2TB 970 EVO runs around $650 plus an adapter that costs $4–$10. You’re already going in so the additional labor isn’t much (loosen the right speaker).

    c) Replace the HDD if you want additional onboard storage or don’t. A 2TB Cucial MX500 is as good as anything and costs $250. If you don’t need the additional storage, don’t bother. No harm in keeping the HDD for storage but transfer speeds to an SSD are over 3x that to a 7200 HDD and yours runs slower @ 5400rpm. I ran those tests this morning and posted in another thread.

    When done, Boot from the USB stick and format the NCMe SSD as APFS, install Mojave. Do not tie to the SATA III as a fusion drive. Although you can, there’s no point. If your total system was under 2TB, run Migration assistant and restore from your backup.

    In Disk Utility, format the SATA III drive APFS if an SSD. Doesn’t matter how you format if you left the original HDD in place.

    Mojave has a good feature in that you can offload your iTunes library to the other drive without using a Sym Link — good reason to keep a drive in the SATA III slot.
  17. rc705 thread starter macrumors newbie

    Apr 13, 2019
    oooh.... so you can keep the original HDD in there? That's what I did in my 2010 iMac. Installed a 2tb SSD and left the 2tb HDD inside and I use that as a Time Machine. Works wonderfully and saved me two times last year. The SSD is literally just taped inside to the side of the iMac, BUT i haven't seen anyone do that with the slimmer iMacs (mine is 2010 so it's the old thick monitor one). If I keep the HDD, where inside do I place my SSD?
  18. mikehalloran, May 5, 2019
    Last edited: May 5, 2019

    mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    I took my 2010 i7 out of service last week. I know these well.

    The NVMe SSD sits in a different place. Yes, you can keep the 3TB HDD in there — heck, you can use it for Time Machine if you like. A local TM drive must be formatted Extended GUID and you can do that after you’re done.

    Unlike the heat pump in the2010, these run much cooler.

    I wrote out complete instructions for making a USB boot installer for any OS from Mavericks to Mojave here.

    This morning, I posted data transfer comparisons that have a direct bearing on this. The SATA III bus is much faster than either an SSD or HDD in that slot as you will see. I didn’t test a 5400 HDD but a 7200rm drive is about 30% the transfer seed of an SSD. I would expect a 5400 HDD to be about 25% that sped or less. For most tasks including Time Machine, this won’t matter.
  19. rc705, May 6, 2019
    Last edited: May 6, 2019

    rc705 thread starter macrumors newbie

    Apr 13, 2019
    Oh, sorry, I think you misunderstood. I'm not touching the NVMe inside, but wanted to swap out the HDD for an SSD only. So I would have a 128gb NVMe, and then a 4TB SSD for storage and working. I watched a video showing how you could swap out the NVMe, but it looked way too difficult. Just removing the HDD only looks a lot easier and manageable. Hm... I'll have to look at other videos now.

    BUT, if I did replace the NVMe and the HDD, would I even have to worry about splitting the drives? Technically they'd be completely blank, and then I would just have to install a fresh copy of Mojave on it using internet recovery, correct?
  20. Fishrrman macrumors P6


    Feb 20, 2009

    Here's what I'd do to "get to where you want to be".
    No one else will recommend this, but I do things my own way.
    I can't guarantee this will work, but this is what I'd try.

    First, you need to create a bootable clone of your existing fusion drive on an external drive (because you're going to WIPE OUT the existing internal fusion drive).

    Then, you need to download LOW Sierra (10.12). According to everymac.com, the 2017 iMacs can boot Low Sierra 10.12.4 -- so the "final version" of Low Sierra should boot and run it just fine.

    Next, you need to create a bootable USB flashdrive with the Low Sierra installer on it. I recommend DiskMaker X for this job.

    Then, you need to boot from the USB flashdrive and use Disk Utility to erase the fusion drive. I would see if it's possible to "break" the fusion using DU -- not sure if it can -- but if it does, you're "mostly home".

    In any case, I'd get the internal drive(s) erased to Mac OS extended with journaling enabled.

    Then I'd try to install Low Sierra onto the internal drive (yes, LOW Sierra!). If the drives are "split", install it onto the SSD. If not, onto the fusion drive.

    Can you get a basic install of LS up-and-running?

    If so, I'd set up a basic account, then boot to the recovery partition.
    Then, I'd see if I could use the terminal to "split" the fusion drive (if it exists). I believe this is a much simpler operation under HFS+ than under APFS.

    If you can get that far, and your drives are now "separated" and running under HFS+, I'd do this:

    Boot from your cloned backup.
    At this point, you have "some decidin' to do".
    You could try to "re-clone" your copy of Mojave on your cloned backup BACK TO the SSD. YOU HAVE TO BE CAREFUL about what "gets cloned", because of the limited space on the SSD.
    CCC will let you "go inside" your backup and "DE-select" stuff that you don't want cloned.
    This would probably include all your saved data and large libraries (movies, music, pictures, etc.).
    You DO want to clone the OS, apps, and BASIC account information (again, you must know what you're doing).
    It can be done.

    Take notes.
    When you get the SSD properly "set up", THEN you can use CCC to "clone everything else" over to the HDD drive.

    The ultimate goal:
    Mojave booting and running from the SSD.
    All data files on the HDD.

    TOO MUCH WORK? (and I kinda agree)
    There's a better way to handle this:

    Buy an EXTERNAL SSD, plug it in, and set it up to become the boot drive.
    For a 2017 iMac, you want something speedy, so I'd suggest a Samsung X5. They are pricey, tho...

    Alternate: get an nvme blade drive and a USBc 3.1 gen2 enclosure, which will give you about 900mbps reads.
  21. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    That might be possible but it's not a given (depends on the month it was made) —no reason not to try it, however because it is the easiest way. If successful, you can skip High Sierra.

    My link shows you how to make USB installers. Either plan on making three or have another Mac available so that you can reuse the same 8G USB stick for each OS.

    A 2017 will run High Sierra, however, even if shipped with Mojave. If you need to do that instead, when you erase and reformat in Disk Utility from the installer, reformat the HDD first—it will format Extended since HS does not support APFS on HDDs. This will split the FD. Now erase and reformat the SSD which will default to APFS.

    In either case, you should be able to download and run the Mojave installer. If not, use a USB installer and install on the SSD. If not already APFS, it will default to that. You can use Migration Assistant to pull your system over but, if your old Mac was over 128GB, not your Applications and files. If System and Applications are under 128G, do that only as you can move your files over manually to the HDD if not enough room on the SSD.

    You may need to do a clean install if your Mac had more than 128GB on it. You can pull your users files from a backup including TimeMachine but applications will likely need to be reinstalled.

    Oh, you do not need a clone if you have a Time Machine backup. You can search and move your files manually if you have to.

    If your system is over 128G, zip your /users/(name) folder and straight copy it to an external. This will save you a lot of time and hassle later.
  22. rc705, May 6, 2019
    Last edited: May 6, 2019

    rc705 thread starter macrumors newbie

    Apr 13, 2019
    Thanks for all the info everyone!! I think what i'm leaning towards to do now is just swap the HDD with a 4TB Samsung SSD and then call it day. BUT I won't be doing it myself because it just seems like a total hassle, so I found a reputable place near me that'll do all the labor, split the fusion drives and install everything for $200. Seems fair. Last time on my 2010 I did all the installs myself, BUT the 2nd time i opened up my iMac I ruined the display cable inside (i didn't realize how FRAGILE it was) and my screen has been shaking and blinking on and off for 6 months. HAHA!! SOOOOO I think it's safe to say I probably shouldn't open up this one (even though I really wanted to try again). This "fusion drive splitting" is probably a warning for me not to touch it and give it to someone who does these frequently. =)

    Then I'll run a small external 4TB WD Passport drive for my Time Machine (unless someone has any advice on other brands or drives).

    And then after all of that is said and done I think i'll buy a RAID enclosure for the two extra 1TB & 2TB SSD drives I have and run it through Thunderbolt 3 for all my video editing needs.

    I didn't know about those little portable Thunderbolt drives! Thanks for sharing that info Fishrrman!

    THANKS AGAIN EVERYONE for your valuable input! It's been very helpful!!
  23. casperes1996 macrumors 68040


    Jan 26, 2014
    Horsens, Denmark
    The comment you're replying to was not in response to the OP, but to a commentor quoting Linus Torvalds having a go at HDDs, and people calling HDDs obsolete spinning rust. They are not.

    First of all; A lot of people have been talking about fusion drives being harder to work with under APFS. I disagree. There's less documentation on the internet, but the duskutil manpages are clear enough and the commands themselves are streamlined.

    Second; I am normally hesitant when it comes to giving very explicit step-by-step guides on something that involves data destruction - Particularly something that can render a drive useless if the user does not know how to recovery in the case of errors. I typically just point people in the right direction, so that if they know enough that they should be messing with it in the first place, they have enough information to find everything else they need - like reading man pages or --help notices.

    That said, I will give you a quick step-by-step, since you've got a clear goal here, but I take no responsibilities for any errors along the way. I'll gladly help you out if you get in trouble, but doing this without direct access isn't always easy.
    And I may get busy with university quickly and be unable to reply for some time - Writing me privately will make me more likely to respind quickly.

    With that out of the way, let's go

    1) Boot from a separate system to the one running on the Fusion Drive. - It doesn't matter if it is Internet Recovery, a USB Installer, or a different installation of macOS. As long as it has diskutil apfs.

    2)write diskutil apfs list
    You'll get the list of APFS containers, physical drives and volumes. Note the container Reference Identifier.

    3) Enter: diskutil apfs deleteContainer [containerRefID] [Optional-new-names-for-separated-drives]
    If no name is entered, they will be named Untitled 1 & Untitled 2. If the name "A" is entered, they will be named "A 1" & "A 2". As far as I know, the command currently does not support a non-numbered naming structure, like "A" "B". The names can be changed later though.

    4) Your Fusion Drive is now split (and entirely empty!). From here on the Disk Utility app can be used as normal to format them the way you want. After the deleteContainer command, the drives are formatted as HFS+. How you proceed from here is up to you, and all can be done graphically. Disk Utility supports formatting both HFS+ and APFS.

    Note: If the above steps are performed from an external installation and not either a bootable installer or Internet Recovery, keep in mind that the split drives do not contain an OS, and you'll have to clone one on, or prepare an installer for installing a new OS as well.
  24. fishbert macrumors member

    Aug 20, 2009
  25. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    That's correct. I think the better bang for the buck would be to replace the NVMe blade with a 2TB plus a 2TB in the SATA III slot the but that's me. Far less housekeeping

    If you're going to replace the HDD with a 4TB 860 EVO, you should consider re-tying it back into the 128G as a Fusion Drive. Seriously, like old Ron Popeil: "Set it and forget it". The SATA III SSD will be 4x faster than the HDD that's there now so any speed hit with large files won't be as great. Unless you are good at keeping up with 'housekeeping' — swapping active files to the NVMe and done files back to the SATA III, what you want to do will result in an overall slower Mac, especially when you factor in the housekeeping time.

    A 2TB boot drive means a lot less housekeeping — not even once a year for me. I don't intend to do so again until OS 10.15 when it will be time a clean install as I clear out 32 bit apps and plugins etc.

    So disagree if you want but that comment makes no sense to me.

    FD APFS is supported only under Mojave. Splitting and removing partitions destroys data and is time consuming but not hard since wiping and reformatting the system is easy. Removing partitions and containers while keeping data intact is quite time consuming as it involves moving data between partitions, alternating between Disk Utility and Terminal Commands till the desired result is achieved. All while hoping that you don't hit the wrong command and have to start from scratch anyway... It's difficult but I've done it on test a drive to see for myself and will never do so again.

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