There is another thread where this is already being discussed:
How to "seriously" securely erase a Late … - Apple Community
Robert Garven wrote:
In recovery mode there is no way to erase and reformat the new Fusion drive to APFS, only extended journaled. I used this link to recreate the fusion drive using terminal How to fix a split Fusion Drive - Apple Support ironically only the second high sierra or earlier commands worked, but the drive ended up being automatically formatted to extended journaled and I don't think Monterey will work or download on that format. I have tried recovery mode OS install several times now with no success. It says an error occurred while preparing the installation. Try again. I tried 10 times now.
What OS installer were you booted when you created the Fusion Drive? It sounds like you were booted into macOS 10.11 El Capitan installer. If so, then once you created the Fusion Drive, you should just quit the Terminal app, and select the "Reinstall macOS El Capitan" option.
If you are booted into the Monterey installer and the command to create the Fusion Drive defaulted to the MacOS Extended (Journaled) file system, then you should just launch Disk Utility and erase the volume as GUID partition & APFS (top option). Quit Disk Utility and select the "Reinstall macOS Monterey" option.
The mistake most people tend to make when reinstalling macOS is rebooting the system after using the Terminal commands and/or Disk Utility. Once you boot into the macOS installer, just launch & quit the various apps until you get to the point where you can select the "Reinstall macOS" option. No need to reboot in between any of these actions. And when using Recovery Mode, it is not always guaranteed which online macOS installer you will actually end up accessing.
You can easily tell which macOS installer you are accessing when you boot to the recovery mode menu where is clearly shows a "Reinstall macOS xxxx" option where "xxxx" is the name of the OS to be installed.
I've even tried to back it up from a time machine external drive, I did on this machine before I migrated it over to my Mac studio and the iMac wouldn't even recognize the drive??
Newer versions of Time Machine no longer backup the system files so it is impossible to restore the bootable macOS system files. macOS must first be installed before you can restore your apps & data from the Time Machine backup.
Plus, I think beginning with Big Sur or Monterey, the Time Machine backups are now on APFS volumes which older versions of macOS are unable to understand. Even when an older OS understands the APFS file system, it is not safe to restore apps & home user folders & settings from a backup made on a later OS since they will contain things the older OS will not understand (it is only safe to retrieve actual data files). Any existing Time Machine backups made to a MacOS Extended volume would have been automatically converted to an APFS volume once you started using Big Sur and/or Monterey.
In the other thread I linked here, I have explained all the necessary steps to recreate the Fusion Drive and reinstall macOS. I even provided clarification for a couple of steps where I discovered the Apple documentation may have been lacking.
I just did this exact same thing within the last six months using the steps outline in the other thread after replacing an internal Hard Drive in an iMac with a Fusion Drive setup. I'm just a bit unsure what file system a Monterey installer may automatically create when creating the Fusion Drive, but I think the Monterey installer automatically uses the APFS file system....that is the only point where I am a bit unsure.
