Back when nobody could afford an HD bigger than 30MB (and DOS couldn't read anything bigger than that), the disk controller built into the hard drive had very little memory, and the heads couldn't read/write as fast as the disk could run the bits past them. It was common to "interleave" the sectors so that as the disk spun the heads read or wrote every other sector (2:1 interleave) or every third sector (3:1 interleave) or some other combination.

A file that was in "logically consecutive" sectors might actually be in physically separated sectors splattered all over the physical disk.

By changing the "interleave" you could speed up the net passthrough for the drive by as much as 5x or 6x over the best you could do without "optimising" the interleave. (Or screw it up by a factor of 5x or more by doing it badly.)

Earliest drives that I made the tweak on mostly peaked at something like 3:1 - 5:1, although one really "wanted to be" at about 7:1, but improvements in the matchup between disk speed and controller capabilities (helped a lot by just a little kick in the internal memory* in the controller, and by much improved head types).

By the time the first 60MB - 90MB HDs were usable (for DOS/Windows), most of them optimized at 1:1 or at worst 2:1, or the manufacturers set a fixed interleave and blocked changes; and people sort of forgot about trying to tweak them.

* The internal memory isn't/wasn't really quite the same as the buffer memory quoted for newer drives, but had the same purpose of providing a cushion for differences in speed for parts of the system that didn't quite match up all the time.

John