Might have solved the problem with the Star Trek Collectors Edition disc I was trying to backup.

Disc 53 (TNG43) has the wrong 3rd episode on it; the same 3rd episode is correctly on disc 56 (TNG46).

This may be why 53 (TNG43) was giving the error messages when trying to make a backup copy, yet played ok on the dvd player.

A replacement disc 53 (TNG43) will be included with Collectors Edition disc 57 (TNG47) issue - I'll see if the replacement copies correctly.