BradHolt wrote:
I launched Disk Utility and it found the external drive but said it was unmounted. I tried to mount it with no success. I then tried running First Aid but that failed. I then ran Restore and that failed, but the disk suddenly mounted, but still didn't show up as an available drive in Finder or Time Machine. I tried to Eject and a message popped up that it couldn't be ejected because it was in use.
The external drive MAY be damaged. How old is it? Can you try it on another computer.
I say MAY be damaged because there are other possibilities. When Disk Utility finds the drive but it shows up unmounted, that can happen when the MacOS detects a problem with the drive and automatically launches fsck to try to repair it. This (fsck) can run for a long time, possibly hours, if this is a backup Time Machine drive with many files. While this is happening the drive remains unmounted and the light blinks indicating drive access is happening. If you somehow get it to mount before fsck completes the disk mount might be unstable due to unresolved errors or damage and very slow, requiring many retries, hence it appears "busy" or "in use" as it tries and retries even the simplest of tasks.
Sometimes a damaged drive will not work on one computer but barely function on another. Can you try another computer somewhere? It might also be due to a problem with the new computer or one of its ports, in which case it would work on another computer. Also, I would try another cable/connector, those are the cheapest part of the chain and sometimes fail. What type of USB cable is this (USB, USB 3, USB-C ...), and is there an adaptor used as well (try a different adaptor).