Re: Floppy drives
I recall on the Apple ][ (please folk, get it right), the disk drives made no use whatsoever of the index hole for timing or locating the first sector, but instead relied on the motor spinning at exactly 300 RPM and reading and writing to the disk at the bit level. This meant making a flippy was as simple as cutting an extra write enable notch on the other side of the disk. (Or installing a switch which bypassed the write enable sensor entirely.)
All disk access was done entirely in software. A carefully timed (40 clock cycle loop) chunk of code waited for a specific patten of 10 bit long "bytes" to pass under the read head. Actual data was read and written with a 32 (8 bit) loop.
To avoid the possibility of a sequence of stored data accidentally mimicking the lead in index "bytes", only 64 of the possible 256 possible 8 bit patterns were originally permitted, and data was written using 5 + 3 bit encoding, meaning it took 3 bytes of space on the disk to store 2 bytes of data.
As it happens it's possible to store 96 (more?) unique bit patterns without upsetting the index byte apple cart, but the tight timing of the write code made it impossible to take apart three bytes and reassemble them into four (6 + 2 bit encoding) on the fly with code alone.
As memory got a bit cheaper, buffering and some trickery with lookup tables, solved this problem, and Apple was able to up it's sector count from 13 to 16 per track.
More exotic coding schemes were introduced by developers to prevent the standard utilities being used to duplicate disks and the copy protection arms race was on.
And then there was all the fun that could be had with 1/2 and 1/4 tracking, the early read/write heads were too "smeary" to permit adjacent tracks that closely spaced, but because ALL timing was in software, spiral tracking was possible.
Oh the joys of MUFFIN, FID, Locksmith and boot-tracing self modifying code with custom ROMs. All for legitimate backup purposes only of course.