At first I thought maybe my external drive was in read-only mode somehow. Because the beeps made me think the files couldn't be modified.
I've never seen anything like this before, especially since the keystrokes are coming through. It's just the 10-keypad hardware being blocked, and only for the 1-5 rating keystrokes, not any other keys like Esc on the keypads, and only by Photo Mechanic, and only when trying to rate photos. The keystrokes work fine elsewhere in Photo Mechanic, even when trying to set the shortcut in the preferences panel. They just don't work when trying to actually use them to rate photos.
Perhaps the biggest help for diagnosing what's causing the issue is the "beep" -- what lines of code in Photo Mechanic produce a beep when a key is typed on a keyboard? One of those code statements is being triggered somehow.