If you are using macOS Catalina and can't see your external drives or camera cards (they appear but don't expand) in the navigator column of contact sheets, or you have trouble ingesting from cards, you may need to make some changes in the macOS System Preferences. Catalina requires more permissions to allow some apps, including Photo Mechanic full access to disks.
Open System Preferences, and select Security & Privacy. Select Privacy.
Select Full Disk Access, then check to see if Photo Mechanic is listed. If so, select the padlock in corner of the preferences window, and then check the box next to Photo Mechanic 6. Relock the padlock. That should bring your drives back to the Navigator column.
If Photo Mechanic isn't available, use the plus (+) button and add Photo Mechanic.