Ok, cracked it. What worked to get beyond step 4 and get GYM working, was to invoke SonicStage's option "Controls->Device/media->Delete All tracks". I guess that made the contents of OMGAUDIO self-consistent (who knows?). I could then run GYM and save orphaned mp3 files on the HD5 to my PC. Thanks to the author of GYM (no really!).