Okay, here's a possibility. Disks wiht different sector sizes depending on whether they are Mac or PC and the Mac mode switch controls the expected sector size. If you have a disk with a PC sector size and your drive is set to Mac mode, then it would likely reject the disk.

This is farfetched, but I have these vague memories of having a batch of MO disks with the wrong sector size back in the 90s -- only that was a Unix vs. PC situation, I think.