This question is related to External drive does not mount after plug off without eject, but the difference is that in my case, the drive did not appear in the Disk Utility application nor in
diskutil list. Furthermore, I had a strong suspicion that the problem was caused by a poweroff while the external drive wasn't (fully) unmounted, because Finder was showing a warning about disconnecting before unmounting for both partitions of the drive.
When I switched it on, the drive would briefly spin up and then spin down again, but none of the software on my mac would confirm that it was connected.
There are lots of (free) tools available online that promise to help with recovering compromised disks, including some that are provided by disk manufacturers. However, in this case, none of the programs I could find appealed to me, for at least one of the following reasons in each case:
- It is only for Windows, or it requires creating a bootable flash drive before you can run it.
- Its documentation does not clearly state that it will be able to find a disk that doesn't already show up on your mac.
I'm asking this question retroactively to share the solution I found. If you know of a different solution that worked in your case, please feel welcome to share as well!