I suppose it couldn't hurt to ensure that Photo Mechanic is allowed full disk access. I have never seen this preference arbitrarily prevent Photo Mechanic from seeing certain folders before, but it has resolved a number of issues around accessing external drives.
1. Quit Photo Mechanic.
2. Open up the macOS System Preferences and go to "Security and Privacy".
3. Under the Privacy tab, select 'Full Disk access' on the left hand side.
4. Enter your administrator password to be able to make changes.
5. Find Photo Mechanic in the list on the right and check the box next to it (if Photo Mechanic is not in the list, select the + sign under the window and add it from the Applications folder).
6. Close the System Preferences window.
If that does not resolve the issue, I'd be curious to know what kind of file system the drive is using.