Which computer systems used hard sector indexing of floppy disks?
On 5 1/4" and 8" disks there is a hole punched into the disk used for optical timing to locate the start of sectors. There were two types of index holes, hard sector and soft sector.
Hard sectored disks have an index hole for every sector boundary; if the disk has 10 sectors, there are 10 index holes, plus usually one additional hole in a half-sector arrangement, to mark the starting sector for each revolution.
Soft sectored disks have a single index hole. This marks the starting position of the first sector for each track and the remaining sectors are either defined in software encoding, or can be located via measuring the rotational time detecting the single index hole for each revolution and doing fractional math to find the timing for the start of each sector based on that.
Steve Wozniak is famous for designing the Apple II DOS to work without using the index hole, and the electronics for it could be excluded in Apple floppy drives to reduce costs.
Which systems required hard-sector indexing, and could not function without it?