Ugh… it was something stupid. When moving files around the other day I must have done something like this.
mv gauntdl.chd /opt/retropie/config/mame-advmame/image/gauntdl/guantdl.chd
Misspelled the target name and renamed the file.
After fixing that brilliant mess-up it still didn’t work but that was because it was saying that the ROM and CHD weren’t a match. That’s at least a problem I can understand.