These days, of course, the idea that 100MB is "a lot of data" is pretty preposterous. Shoot H.264 video at 1080p and 60fps for four seconds, such as on an iPhone 6, and you've generated a hundred megs of data.
A little over 20 years ago, however, when Iomega introduced the original 100MB Zip disk, that was staggeringly huge for a removable disk. The wildly more common 3.5-inch floppies held 1.4MB. For context, the entry-level PowerBook 150, introduced in the same year, had a 120MB hard disk, and the base configurations of even 1994's server Macs came with hard disks that were only five times the capacity of the Zip disk.
The humble Zip disk, then, was a kind of de facto successor to the ubiquitous high-density 3.5-inch floppy. You had to buy a special drive to mount it in, because although they had about the same footprint as a regular floppy disk, they were much thicker. In fact, Zip disks had a lovely chunky, seemingly hugely robust quality compared to normal floppies.
(Drives for the competing 120MB SuperDisk format could read regular 3.5-inch floppies backwards-compatibly.) So similar in size were Zip disks to 3.5-inch floppies, however, that you could try to jam a floppy into a Zip drive, and since this could damage the Zip drive's heads if it tried to mount it, there was a security system built into the disks: If the drive didn't detect this retroreflective spot on the underside, it wouldn't even try to mount it.
Later, when 250 and 750MB Zip disk variants appeared, this spot was used to identify the capacity of a disk, since while newer drives were backwards-compatible, older drives couldn't mount the higher-capacity disks. (An alternative reading of history is that Iomega introduced this retroreflector spot so that it could quash the market for cheap, unlicensed third-party compatible Zip disks.)
So popular did the Zip disk become that just a couple of years after its introduction Apple started offering internal Zip drives as an option in some Macs.
Zip gained popularity especially in the design industry, and indeed I first remember actually handling Zip disks when I did an internship at my local council's in-house design department. I soon saved up and bought an external Zip drive of my own, though. It used the old SCSI connection, and while I also had a SCSI scanner — a cheap, end-of-stock clearance Umax which came with a full version of Photoshop 3.0, something I religiously upgraded for many years to come — I liked having the option of passing that SCSI connection through the Zip drive and attaching my SCSI terminator block to the scanner or connecting the scanner to the Mac first and taking advantage of the built-in SCSI termination switch inside the Zip drive.
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