I am unable to recover my Fusion Drive on my iMac

The Apple Diagnostics confirm you have a failing drive.....most likely the hard drive portion of the Fusion Drive.

When posting the DriveDx reports, you need to copy the text from the report into the system clipboard, then paste the contents of the clipboard into the main field of the "Additional Text" window.

If either the internal SSD or HD appear as mounted volumes within macOS, then use the Finder or Disk Utility to eject them as soon as you can log in (especially the volume associated with the hard drive) to see if that prevents the reboots. You can also try preventing macOS from automatically mounting the volume on the internal hard drive by using the instructions I linked in my previous post. From everything you have mentioned in this thread, it may not be possible to do more than eject the internal drive since preventing macOS from mounting the volumes requires being able to communicate with the internal drive in order to get the UUID of the volume which may be impossible if the drive failure is severe enough to cause a system reboot while booted from an external drive.

If you are unable to eject the internal drives or edit the "fstab" file to prevent the volume(s) of the internal drives from automatically mounting (using the linked instructions), then there is nothing to do except to open the iMac (not easy) and disconnect the failing drive to confirm you can now run the iMac from the external SSD without issues.

Edit: I should also add.....if the internal hard drive is causing this problem now, then there is a good chance as the drive failure gets worse that it may even affect the ability to even boot from external media. However, if the internal drive is not being actively accessed, it may slow down its deterioration.

Thanks for posting the DriveDx reports.

The Seagate internal hard drive is in really bad condition because half of the report is missing (no health attributes shown) and the report reveals that the hard drive is reporting only a size of 137GB instead of 3TB. This can easily explain why you are having issues even when booted to the external macOS drive.

Unfortunately the only option to continue using this iMac would be to physically disconnect the internal hard drive which requires opening up the iMac. Then testing how the iMac works while booted to the external drive to confirm the internal hard drive is the only hardware issue. My earlier suggestions to eject the internal hard drive or prevent the volumes from automatically mounting no longer apply since the internal hard drive in such a bad state.

It is not easy to open up the iMacs, but at this point you really have nothing to lose since the iMac is not usable even when booted to the external drive. You can find video instructions on the OWC website which show how to replace the internal hard drive with a third party SSD, and you may also find some written instructions & picture guides on iFixIt. The OWC videos are located on the product page for their SSD upgrade kits.

One of the drives is failing or has failed. Not always, but generally it is the HDD part of the Fusion Drive that goes first.

To check the health of the internal drives, you can use DriveDX.

https://binaryfruit.com/drivedx

Sadly it sounds like the failing or failed drive is causing an issue while running from the external SSD. If you can not access and erase the failing drive that is causing the problem, then the drive needs to be replaced.

So you had no issues installing macOS to the external SSD?

You only have a problem with the iMac rebooting while running from the external SSD? What are you doing with the computer when these reboots occur?

Did you unmount the internal drives or eject them? Ejecting the drive occurs when you click the arrow next to all of the mounted volumes for that drive within the Finder. You want to eject the drive since macOS should not be able to communicate with them until a reboot (not sure about wake from sleep). The macOS Finder basically combines unmount & eject in one operation if all volumes are unmounted from the drive.

You can use the information in the following article to prevent macOS from automatically mounting the internal drives which should mostly eliminate them as a problem unless the hardware issue with the internal drive is so bad it is causing a problem even when the internal drive has been "ejected".

Prevent a volume from mounting at startup - Apple Community

You should run DriveDx as suggested previously and post the health reports so we can examine them to see how bad the internal drive's failure may be.

You should also run the Apple Diagnostics to see if any hardware issues are detected. It is possible you have some other hardware issue causing a problem.

Did you test booting from the external drive to see how the iMac worked before installing any third party software and before restoring from a backup? And before connecting with your AppleID & iCloud? Try booting into Safe Mode to see if that makes any difference.

Disconnect all external devices in case one of them is causing a problem & make sure the external SSD is connected directly to the computer.

Un-mounting the internal drives will not help because they will just keep re-mounting.

I might try erasing and reinstall the macOS on each drive separately to see if that helps.

If not, then replacing on having the failing/failed drive replaced is the only solution.

Agree with @den.thed that the hard drive has failed. The Fusion Drive setup had issues with the larger drive which would be the hard drive.

If this is a 2012+ Mac, then you can use an external USB3 SSD as an external boot drive for running macOS which should be achieve similar speeds (about 500MB/s). This is the easiest solution to keep using the iMac as long as the failure of the internal hard drive does not affect system performance since it can be expensive to have someone replace the failed internal hard drive with an SSD upgrade....it is also difficult to open up & replace the failed internal hard drive yourself.

You're welcome. I tend to agree, based on age and the lack of supported macOS version upgrades.

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