Maybe your internal drive on the Amiga 1200 was High Density? Probably not if you never opened it up before. Usuaully one format works with all the Amigas. Unless you used some kind of utility to compress the disk or use something like Quarterback, then there shouldnt be a problem.
Also, maybe age and some kind of magnetic field ruined the disks?
Possibly.... Your disk drive in the Amiga 500 could have went bad or is bad... :-)
Just my thoughts.
