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I have a 2013 iMac (27" 3.5GHz i7, model id iMac14,2, EMC 2639) that has a Fusion drive. I want to replace the Fusion's internal HDD with an SSD. My understanding is that I should split the Fusion drive before performing this.

I referred to the article https://www.macworld.com/article/2015664... which discusses the use of diskUtil and Core Storage to get the GUIDs and split the drive.

However, invoking disktutil coreStorage list returned ‘No CoreStorage logical volume groups found’. I then tried diskutil list, which returned more information, apparently showing the fused drives as ‘synthesized’. I read a bit about this in other discussions, and it seems like this is perhaps a Mojave/APFS version of a Core Storage Fusion config.

I’m wondering what I need to do to split my Fusion drive. Is there something else to use, or do I not even need to split the HDD/SSD before replacing the HDD?

Here is the output from diskutil list (I'm booted from an external drive named "External Drive". I think the first three entries are for my internal Fusion drive, and the last two entries are for the external drive:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk2         3.0 TB     disk0s2

/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *121.3 GB   disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                 Apple_APFS Container disk2         121.1 GB   disk1s2

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +3.1 TB     disk2
                                 Physical Stores disk1s2, disk0s2
   1:                APFS Volume Preboot                 28.7 KB    disk2s2
   2:                APFS Volume Recovery                20.5 KB    disk2s3
   3:                APFS Volume VM                      2.1 GB     disk2s4
   4:                APFS Volume Macintosh HD            9.6 MB     disk2s1

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2:                 Apple_APFS Container disk4         4.0 TB     disk3s2

/dev/disk4 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +4.0 TB     disk4
                                 Physical Store disk3s2
   1:                APFS Volume External Drive          81.6 GB    disk4s1
   2:                APFS Volume Preboot                 45.7 MB    disk4s2
   3:                APFS Volume Recovery                512.5 MB   disk4s3
   4:                APFS Volume VM                      17.2 GB    disk4s4

Thanks.

-Allan

  • Mojave does no use Core Storage. This have been replaced with APFS. If you remove the Fusion's internal HDD then you also remove the Fusion's internal SSD. Basic the HDD and SSD are housed together. It would help if you posted the output from diskutil list. Also, what is the size of your iMac screen? – David Anderson Dec 23 '18 at 18:24
  • Hi David. Thanks for your reply. I updated my original question to include more system spec details about my iMac, and I also included the diskutil list output, with some comments describing the the volumes I'm using (internal + external). -Allan – Allan Dec 24 '18 at 0:33
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I did this earlier today on an iMac. Boot to a Mojave USB boot installer. Then Utilities → Terminal. The commands are along the lines of:

diskutil apfs list

Then make note of the “virtual disk” that is the “Fusion Drive”. This is made up the PCI Flash and the other spinning drive. You can see how the text output of the drive structure is hierarchical.

Let’s say that virtual container disk is identified as “disk0”

diskutil apfs deletecontainer disk0

This command will split your fusion drive, and leave you with two new drives to format via Disk Utility (which you can run in the same USB Booted environment you’re already in).

Format them (the SSD and the other drive) as whatever you want.

I recently did this prior to replacing the slow spinning disk that comprised the 1TB of my Fusion Drive. Replaced that with a 500GB SSD which is SATA3. Installed Mojave onto the 24GB PCI “Blade” Flash and formatted the other SSD.

I then store User home folder on the larger SSD, and macOS and other apps run off the faster PCI-based SSD.

  • Circling back to this. Installing the macOS to the 24GB "blade" ssd was dumb. The final straw was when I had 6GB free and needed 14GB to run an incremental Mac OS update. So, if anyone out there is thinking of this with iMac that has a 1TB fusion drive, just install your whole system to the "slow" SSD you installed. – shadowmoses Apr 3 at 22:01
  • I've swapped out the mechanical HDD in an iMac both ways: (1) Pre-splitting the existing Fusion Drive via Terminal (as I explained in my original answer) and (2) Forgetting to do this before I opened up the iMac. For scenario #2, I don't remember each step exactly, but the system saw the new SSD fine, and booted with the cloned image. I think I then formatted the remaining Apple flash via Disk Utility. – shadowmoses Aug 7 at 17:08
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You do not need to split the Fusion Drive before replacing one of the components. The act of changing the hardware will 'break' the logical volume, and the OS will then treat the two devices as separate.

You will have to erase/re-partition the remaining device.

Of course, splitting the Fusion Drive in this way will render the data on it inaccessible, so you must have a backup to restore your data to each volume as you wish.

  • Thanks for your reply. A concern I have about simply changing the hardware without pre-splitting the Fusion drive is that I've read a number of articles/comments in which it is said that by not pre-splitting, the system will be in a "confused" state after the hardware swap (though it's not clear to me what the implications of that are). – Allan Dec 24 '18 at 15:52
  • Cloning disk images is done all the time from one hardware config to another without incident. If you want to be super-safe, you could just clean install the OS onto the new SSD and migrate everything from your backup. But I've done this, and I just cloned my old Fusion drive onto the new SSD. – benwiggy Dec 24 '18 at 17:14
  • I read about 2 other slightly more tedious ways to split a fusion drive. One says to delete the Logical Volume Group. Another says to delete the Logical Volume and the Logical Volume Group. Do these methods yield exactly the same results? Thanks. – Lost Explorer May 9 at 2:40

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