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My iMac 21.5 inch 4k (late 2015) has been abnormally slow during regular use, so I used Drive Dx to diagnose it. I get a "failing" status on the report, which is understandable.

The first thing I did after seeing this was make a backup using Carbon Copy Cloner. Note that prior to this backup process, the 24 GB "blade" SSD and 1TB HDD were used together as a Fusion drive (1.02 TB) and were not split. It showed as 1 drive under About This Mac > Storage, and under disk utility but I could see the 24 GB SSD using diskutil list.

My external SSD now has a full copy of my computer, which is good, but I'm not quite sure how to go about restoring it. If I remove the 1TB HDD and replace it with an SSD, it wouldn't make sense to do diskutil resetfusion and make use of the 24 GB blade SSD, correct? Here is what I would normally do for a clean install:

  1. Replace 1TB HDD with the appropriate SSD
  2. Go into recovery mode (Cmd+R or Cmd+Opt+R), erase and format the SSD to APFS or MacOS extended (journaled)
  3. Choose "Reinstall macOS"
  4. When macOS boots, locate the SSD under "Transfer information to this Mac". Something like this:

Screenshot of the 'Transfer Information to This Mac' window

There seems to be missing intermediate steps since the external backup is a clone of the fusion drive which contains the data on the 24 GB blade. By the way, I'm pretty sure the external backup is not bootable, since I just used CCC's default backup task for copying the filesystem. I will definitely experiment a bit once I get the kit and the SSD, but I wanted to prepare for it and get some advice in advance.

Note: I did find this post about cloning a Fusion drive and restoring to a new SSD, but was still unsure of what happens to the data from 24 GB blade.

Update: Just make a time machine backup to an external ssd, install the new internal ssd and restore the backup after macOS installs. The 24gb blade ssd shouldn't cause any issues in the restore process since they are no longer "fused" I think. I've heard people booting from that 24gb ssd and storing data on the newly installed ssd so that's definitely an option instead of leaving it unused(?)

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  • What OS is/was on it? Have you tested the CCC isn't bootable? They usually are [though these days it can depend on what OS it was.] Up to Mojave, dead easy. Since then, the system volume tends to make things generally tougher.
    – Tetsujin
    Dec 9, 2022 at 18:10
  • I ended up using time machine to back up to external SSD. Then I inserted the new 2.5 internal SSD, installed macOS and restored from the time machine backup. The 24gb blade ssd is still there and not in use, but that didn't cause any anticipated issues in the restore process.
    – bruteforce
    Jan 25 at 1:35

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