Hard drive in external enclosure won't mount

One thing to look for is macOS wants to decide if a disk is OK before mounting it. Look for a fsck process in Activity Monitor - be sure you show all processes.

If that is running, Disk Utility and Finder won’t do anything, so you may need to wait until that finished (hours or more in some cases) to be presented with the data in read-only mode or read-write mode.

If you just care about reading the data, use a tool that interrupts this mount and just lets you see the disk in read only mode.

The free demo of Data Rescue by Prosoft Engineering is the perfect tool for this job - no need to pay for it unless you decide it has failed and that tool will get you the files back pronto.

• https://www.prosofteng.com/mac-data-recovery

I have no ties with Prosoft - just every time I’ve spend money with them, the value has been amazing. You could also choose a good vendor like Drive Savers and just send them the drive for a free quote if you see IO errors in the console log while the filesystem check is running or you hear sounds it’s failing.