I think I have literally tried everything & searched everywhere. I just want to erase my old computer. I'm have no luck whatsoever & from what I've seen, it's an issue for many. Ideas & suggestions, other than hit it with a hammer would be welcome.
iMac 21.5″, macOS 10.13
You can force unmount a drive by running the following Terminal command:
diskutil unmountDisk force /Volumes/VOLUMENAME
Replace VOLUMENAME with the name of a volume on the disk you are trying to unmount.
Following this, attempt to Erase/Partition the drive again in Disk Utility. Using the above command can interrupt file read/writes, which can cause file corruption. However, since you are erasing the drive anyway, this doesn't really matter in your situation.
https://apple.stackexchange.com/questions/105673/disk-utility-wont-unmount-hd-for-formatting
https://www.amsys.co.uk/disk-utility-tip-fix-couldnt-unmount-disk-errors/
You can force unmount a drive by running the following Terminal command:
diskutil unmountDisk force /Volumes/VOLUMENAME
Replace VOLUMENAME with the name of a volume on the disk you are trying to unmount.
Following this, attempt to Erase/Partition the drive again in Disk Utility. Using the above command can interrupt file read/writes, which can cause file corruption. However, since you are erasing the drive anyway, this doesn't really matter in your situation.
https://apple.stackexchange.com/questions/105673/disk-utility-wont-unmount-hd-for-formatting
https://www.amsys.co.uk/disk-utility-tip-fix-couldnt-unmount-disk-errors/
error 69888 Can't unmount disk