Unable to mount a SSD hard drive on macOS Catalina
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 :)