> I never read how the zip drive can achieve 100x or more capacity than the floppy. What is the engineering feat that allowed this?

Improvement in production processes and materials (e.g. magnetic coatings) allowing smaller tracks and smaller more precisely positioned heads. The 3 1/2 floppy dates back to 1983, the high-density 1.44MB to 1986, the Zip drive was released in 1994.

A “super high density” 20 MB floppy had already been attempted in 1990, and the LS-120, which had the exact same dimensions as a 3.5” floppy (and could read those), launched in 1996, so it was not really exceptional at 6 doubling in 8 years from the 1.44MB floppy.

Also it was expensive, part of that was the lower scale and lack of competition but the increased production requirements were also a factor, Zip drives and media had tighter tolerances.

The click of death was because when the head got misaligned the drive would return it to the home position, if part of the drive had failed the head would never realign so the drive would keep trying, producing a characteristic clicking sound. HDDs can develop the same, but it’s less common than it was on Zip drives. The tighter tolerance were most likely a factor, it was more likely for a zip to age out of tolerance and develop terminal misalignment.

> Improvement in production processes and materials (e.g. magnetic coatings) allowing smaller tracks

Improvements in coatings improve the data per track, but no improvement was needed for increasing the amount of tracks. On a 1.44MB drive there are 100 000 bits per track, but only 80 tracks per side. Or, in other terms, the length of a single bit along the track (on the innermost track) was ~1.2µm, and the width of that same bit, sideways to the track, was ~200µm, for an aspect ratio of 166:1. As far as the media was concerned, roughly 10:1 aspect ratio would have been more than enough, or a normal 1.44MB floppy could have supported more than a 1000 tracks per side.

The limiting factor was that old floppies had no way for the head to follow the track, it was just indexed into a fixed position by the drive mechanism. This meant that the tracks had to be ridiculously wide to support all the possible misalignment on both the reader and the writer. To improve track density, what was needed was some mechanism to make the head locate the tracks and follow them as the disk rotated under them. Iomega solved this by etching shallow concentric circles for the tracks on the surface of the disc. These rings were essentially invisible for the magnetic head, but allowed a separate laser to pick the up and follow them.

The real click of death was when this was due to a catastrophic failure - say, one of the heads had become completely dislodged and was suddenly hanging loose. Then, every single cartridge you inserted into such a drive would be damaged. If you then took that cartridge and inserted it into a fully working drive, it had a good chance of subsequently destroying that drive.

Steve Gibson has a good site with historical information from the time when these drives were still marketed and sold: https://www.grc.com/tip/codfaq1.htm