After upgrading my version of Mac OS, I am having trouble reading some DOS floppy disks. The first part of the file appears OK but the last part is garbled. What is wrong?

This appears to be an issue with PC Exchange 2.1.1, which is included in Mac OS 7.6 and Mac OS 8, and the way some Window computers format floppy disks.

Floppy disks that have this problem have FAT16 written to their boot records, but these disks actually contain FAT12 directories. PC Exchange 2.1.1 reads and checks the FAT field, determines it is a FAT 16 format, and reads the data incorrectly from the clusters on the disk. However, PC Exchange 2.0.7, and earlier versions ignore the field in the boot record.

WARNING!
When PC Exchange 2.1.1 writes anything to a disk that contains a FAT12 format, but thinks it's a FAT16 format, it can corrupt all data on the disk. This can happen even if the Finder creates the file, as it does when the FINDER.DAT file is created or updated on an unlocked floppy disk for use in Mac OS environment. Therefore, just inserting and browsing the contents of an unlocked DOS formatted disk using Mac OS can corrupt the DOS disk.

Not all DOS formatted floppy disks have this problem, it depends on what was written in the boot record by the DOS computer.
Two current workarounds are:

• to use PC Exchange 2.0.5 or 2.0.7 on systems with PC Compatibility Cards, which ignores the FAT16 or FAT12 field in the boot record.
• reformat the disk as a Macintosh disk, then reformat the disk as a PC disk.