At secondary school I was dealing with Agat computers at school computer class (exactly, this was Agat-7), clones of Apple II series. Their 5″ diskette drives were also clone of Steve Wozniak and Shugart disks of Apple II, but with some misdesigns, as absense of zero track sensor. To seek to zero track (after reset or I/O error), it performed 160 steps of stepper motor, unconditionally. This caused gradual disbalancing of head positioner by regular hitting into seeking limiter (with a loud machinegun-like burst sound). As result, at my final class at the school, the computer class comprised of 3 groups of 3 computers each, so, one computer can easily read diskettes written by another one in the same group, but hardly from another group :( And, this issue was known over all Agat park.
To solve this, a program was developed (by a Moscow firm close to the mill produced these computers) to recover unreadable diskettes. It tried to read all sectors of a track in the following manner:
- It applied short-timed affections to stepper motor to make it turn to an angle less than a single step, to position read-write magnetic head to offsets like "1/4 of track interval". A sector was being read multiple times with different head offsets, to find a best reading.
- Besides (1), multiple readings were applied to find unstable bits and statistically find a most probable variant.
There were no algorithmic recovering due to lack of redundant bytes (as most current drives utilize Reed-Solomon or analog), only a single XOR CRC byte, but this was enough in most cases. After reading, user could command the program to write a track back to diskette, according to the current drive specifics.
All this weird magic was specific to so-called 140KB Shugart drive (35 tracks of 16 sectors of 256 bytes). Just after my graduation from the school (1990), it had got a few computers of a new model, with 840KB drive that was saved of this issue due to adding of zero track sensor. (But this was time of a speedy decline of Agat usage due to import of MSX and IBM PC series.)
I hope this will answer your question, despite this was not PC specific.