Corrupt internal HD causing Finder and Disk Utility to crash

Hello,

A couple of years ago I migrated from using my internal HD to an external Samsung T5 SSD. At the time I didn't wipe/Erase my internal HD after I stopped using it.

Fast forward to now and partly in trying to fix some Time Machine/Migration Assistant issues, and partly just to tidy up, I attempted to Erase the internal HD. I believe this process got interrupted/failed part of the way through, as Disk Utility froze and I had to manually shut down (hold power button) and switch back on.

Now I believe the disk has corrupted, as Finder and Disk Utility both freeze as soon as I open them. I also can't get into Recovery mode anymore from startup. Also when I hold 'alt' on startup and choose a Bootable USB disk, the loading bar takes ages to get to the end then hangs, and does not do the Recovery menu. Trying to get into Internet Recovery Mode does the same.

I have gone into Terminal and tried to Erase the disk and/or Volume but it says the volume cannot be unmounted. I have tried a 'diskutil repairVolume disk1s5' and this still running after 4 days (Terminal says 'Repairing file system' with the loading backslashes). In Activity Monitor the 'diskutil' process is consistently at 0.3%, however I would have thought the command would have completed/failed on a 1TB drive within 4 days(?) so can't help but think nothing is actually being done.

Please does anybody know where I can go from here, without manually opening the iMac and unplugging the drive?

Any help would be very much appreciated.

Many thanks