Unable to mount a SSD hard drive on macOS Catalina I attached a brand new Seagate USB SSD drive (Backup Plus Slim) to a MacBook Pro running 10.15.2. It mounted the first time, I could see it in Finder, and was able to run the Mac start up program on the drive. However this did not seem to work, so I disconnected the drive. I thought I ejected it properly but not absolutely sure.

Then when I plugged it in again, I could not see it in Finder.  I also tried on a Mac mini running 10.14.6 but could not see it in Finder either.  Back on the MacBook Pro, I tried Disk Utility. It found the disk, but it was greyed out and not mounted. When I tried to mount it, I saw the following error:

Could not mount “disk2s2”. (com.apple.DiskManagement.disenter error -119930868.)


Doing a search on Google, I found this discussion, which indicates it is a Catalina problem. I could not see any useful answer.

I have tried waiting for hours for it to fix itself and rebooting my machine, but it appears that the disk has become damaged. How do I fix this?
Exactly my issue, but luckily, the solution was simple! Take a coffee and wait. Seems like Catalina does a thorough disk check whenever you try to mount some sort of disk. For example, my HDD had around 500GB of data and it took around 5 minutes before it was automatically mounted and working as expected. In the meantime, I was unable to mount and to eject the disk (the error was the same, the disk was in use by another process). Simply plug in the disk and let the OS do the check it needs (some other users might know more about what is going on in the background and feel free to edit my comment to extend the knowledge). Patience, it will mount itself automatically after it finished all the background checks :)