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User profile for user: Jumlungu
Jumlungu Author
User level: Level 1
4 points

Split Fusion Drive issue

I have a 21,5 inch 4K iMac with 1TB Fusion Drive but since I reinstalled (and messed about with the partitioning) my iMac takes 4 minutes to boot and once inside everything is slower than a tortoise and no fun to play with.


I've been looking for solutions for weeks and have now stumbled on a support article called How to Fix a Split Fusion Drive where it states "If your Fusion Drive appears as two drives instead of one in the Finder, it's no longer working as a Fusion Drive." This is the case with my Mac.



The article goes on to say:

  1. Choose Apple () menu > About This Mac, then click Storage. 
    • If you see a drive labeled Fusion Drive, your Fusion Drive is working and this article doesn't apply to you.
    • If you have a Fusion Drive that has been split, you should see two drives. One of them should be labeled Flash Storage, with a capacity of 24GB, 32GB, or 128GB. The other should be at least 1TB.

which is also the case with my iMac



It goes on to say that this can be fixed by following these steps:


  1. Turn on your Mac, then immediately press and hold Command-R to start up from macOS Recovery. Release the keys when you see the Apple logo or spinning globe.
  2. When you see the macOS Utilities window, choose Utilities > Terminal from the menu bar.
  3. Type diskutil resetFusion in the Terminal window, then press Return.
  4. Type Yes (with a capital Y) when prompted, then press Return.


Only ...

Terminal comes back telling me that I have no SSD



I can't make head or tail of this and my iMac is so slow I could kick it's teeth in :-(


Can anyone shed some light on this?


Here's the System Report screenie:


And here's the Storage screenie:


iMac, OS X 10.11

Posted on Oct 29, 2019 11:59 PM

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User profile for user: Jumlungu
Jumlungu Author
User level: Level 1
4 points

Posted on Nov 1, 2019 2:43 AM

I'm using Mojave and I've solved the problem. Here's what I did:


  1. Boot into recovery
  2. Open Terminal
  3. Type diskutil list and press return
  4. Write down the identifiers of your disks. In most cases it's disk0 and disk1 and press return
  5. Force unmount the first disk by typing diskutil unmountDisk force /dev/disk0 and press return
  6. Force unmount the second disk by typing diskutil unmountDisk force /dev/disk1 and press return
  7. Type diskutil cs create Macintosh\ HD disk0 disk1 and press return
  8. Type diskutil cs list and press return - this will display a long alphanumerical code after " Logical Volume Group" looking something like this:
  9. Type diskutil cs createVolume (your logical volume group code) jhfs+ Macintosh\ HD 100% and press return
  10. Once completed close Terminal, choose reinstall MacOS and your Mac should start from Fusion Drive when finished!


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User profile for user: Jumlungu
Jumlungu Author
User level: Level 1
4 points

Nov 1, 2019 2:43 AM in response to samtenor

I'm using Mojave and I've solved the problem. Here's what I did:


  1. Boot into recovery
  2. Open Terminal
  3. Type diskutil list and press return
  4. Write down the identifiers of your disks. In most cases it's disk0 and disk1 and press return
  5. Force unmount the first disk by typing diskutil unmountDisk force /dev/disk0 and press return
  6. Force unmount the second disk by typing diskutil unmountDisk force /dev/disk1 and press return
  7. Type diskutil cs create Macintosh\ HD disk0 disk1 and press return
  8. Type diskutil cs list and press return - this will display a long alphanumerical code after " Logical Volume Group" looking something like this:
  9. Type diskutil cs createVolume (your logical volume group code) jhfs+ Macintosh\ HD 100% and press return
  10. Once completed close Terminal, choose reinstall MacOS and your Mac should start from Fusion Drive when finished!


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User profile for user: Jumlungu
Jumlungu Author
User level: Level 1
4 points

Oct 31, 2019 12:00 AM in response to Jumlungu

I read that I should try the OS Sierra option in the "Reset Fusion" Article I mentioned above so I booted into Recovery, typed diskutil list in terminal and it came back with 22 disk identifiers. How can this possibly be?? (Unfortunately I couldn't make a screen capture so I then rebooted into Mojave opened Terminal and entered that command again so that I could capture the image but then it came back with "only" two disks. This is very confusing



Right now I have two options to try:

  1. samentor's "bootable USB"
  2. The Sierra option for Reset Fusion


First I have to go to the dentist :-(

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User profile for user: samtenor
samtenor
User level: Level 4
1,684 points

Oct 30, 2019 2:46 AM in response to Jumlungu

Did you try first aide or repair prior to the "reset fusion" ?

If one of the drives fails, you might want to contact Apple for services.


If both drives are still OK and you want to tackle the issues.. read on..


I have played with "Fusion drive" and found "rebuild" the fusion drive can be hit or missed.


First thing first -- I hope you have Time Machine Backup, because the following steps will delete all the data existing on the current 2 drives.


According to "my" experience, I found I have to use "bootable" USB MacOS to access the "terminal" with better success...


  1. Make a bootable MacOS of your choice 10.14 or 10.15 etc. (likely, you have to use different Mac to make this USB drive). Google it, making USB bootable drive might take extra time, but worth it.
  2. plug the USB drive in the Mac you want to fix
  3. Boot from USB Mac OS (by holding Option Key) to access Disk Utility
  4. Erase both drives with APFS format
  5. Use terminal command "diskutil resetFusion" to rebuild the Fusion drive [if this failed, repeat step 4, and format both drives with Mac OS Extended (Journaled) -- do not worry about this format, MacOS will automatically change into APFS during installation]


After, you can re-install the MacOS, then follow the prompt for TimeMachine to restore.


Good Luck

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User profile for user: Community User
Community User

Nov 3, 2019 2:13 AM in response to Jumlungu

Carbon Copy Cloner is an excellent backup tool with a good reputation. It creates genuinely bootable backups unlike TM which relies on a working Recovery volume to do a full restore. The latest version of CCC works well with Catalina. I actually use TM all the time but also perform regular CCC backups just in case of possible corruption of the machine’s built in HD.

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Split Fusion Drive issue

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