09-23-2019, 06:29 PM
On standard double sided disks/drives, the tracks of the second side are at a slight physical offset. This has to do with the way double sided drive head assemblies work - you can't just put both heads precisely right on top of each other.
The problem comes in that many single sided disks, especially in Apple II and C64, were used as "flippy" disks. Instead of having two heads, users used both sides by manually flipping the disk over. These disks do not have that offset. So you can't just read the back side of a "flippy disk" with the second head of a normal drive.
The problem here is that many IBM PC style drives also look for an Index hole. Systems like the Apple II ignored the index holes. If you flip that disk over like the Apple II users did, the IBM PC style drive will not find an index (unless someone punched a second hole) and may refuse to ever enter a "ready" state, preventing reading or writing.
The problem comes in that many single sided disks, especially in Apple II and C64, were used as "flippy" disks. Instead of having two heads, users used both sides by manually flipping the disk over. These disks do not have that offset. So you can't just read the back side of a "flippy disk" with the second head of a normal drive.
The problem here is that many IBM PC style drives also look for an Index hole. Systems like the Apple II ignored the index holes. If you flip that disk over like the Apple II users did, the IBM PC style drive will not find an index (unless someone punched a second hole) and may refuse to ever enter a "ready" state, preventing reading or writing.