I recently bought a used 2017 iMac which came with 2 physically separate drives it seems. One is the internal 32GB Apple PCI-Express Internal Physical Disk and the second one is a 256GB Samsung SSD (SATA Internal Physical Disk). It seems the previous owner had the latter swapped in for a HDD (so they were using it as a fusion drive).

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Now when I am trying to erase both these drives and install Catalina from Disk Utility menu, If I chose the Apple 32GB, it doesn't allow me to install other software on the bigger 256GB because it is only allowed on the disk with MacOS installation.

How should I set up my system so that it can take more software installations [I am thinking of using the 256GB SATA to have the MacOS but it makes my 32GB drive redundant]?

I want to be able to use the entire 256GB +32GB regardless of which one has the operating system.


  • A fusion drive used the small SSD to act as cache for the larger HD. It makes no sense to try rebuild 2 SSDs as a fusion, just abandon the 32GB.
    – Tetsujin
    Apr 29 '20 at 17:17
  • I just got this working on my iMac with a 128GB and a 1TB SSD. it’s no longer a fusion drive, but you can combine the two. I’m way from my desk and working off an iPhone. Give me some time to return and I’ll write it up. Also, if you need me to do this remotely, I can assist
    – Allan
    Apr 29 '20 at 17:22
  • @Tetsujin I understand but is there any way that I can use the smaller volume (32GB) for any kind of file storage as well? Just want to be able to make the most of the space while not having to open this machine up and increase the storage. I know 32GB is not a lot but yea.
    – bit_by_bit
    Apr 29 '20 at 17:23
  • Keep it as a spare drive … for small things, perhaps. I have bigger SD cards. My phone has more storage.
    – Tetsujin
    Apr 29 '20 at 17:24
  • @Allan I am away from the system too. but yes pointers would help. I am intrigued if two SSDs can be fused and used as one. Esp. if one is Apple's original PCI-Express Internal Physical Disk and the other one is Samsung SATA? Thanks.
    – bit_by_bit
    Apr 29 '20 at 17:27

I think I have what I needed (a combined drive) all of it available for use. It is now listed as a Fusion drive even though both its components are SSDs. Here are the steps I used to combine the 2 drives.

If you're using macOS Mojave or later

- 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. When you see the macOS Utilities window, choose
   Utilities > Terminal from the menu bar.
 - Type diskutil resetFusion in the Terminal window, then press Return.
 - Type Yes (with a capital Y) when prompted, then press Return.
 - When Terminal indicates that the operation was successful, quit
   Terminal to return to the macOS Utilities window.
 - Choose Reinstall macOS, then follow the onscreen instructions to
   reinstall the Mac operating system. Your Mac restarts from your
   Fusion Drive when done.

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