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.
Don't let's be coy about this: buying a Zip drive--which usually came bundled with one disk--was not a cheap enterprise, and nor were the individual disks trivially expensive either. Yes, they were cheaper--and more convenient--than external hard disks, but they were significant investments nevertheless. Today, when the most popular USB flash drive on Amazon is a $15 SanDisk Cruzer that stores 320 times the original 100MB Zip disk, we have a pretty blasé attitude to storage, but in the '90s, you carefully counted the kilobytes when saving a JPEG out from Photoshop, because the literal cost of storage was so high.
Later, when I bought a G4 Cube, I also bought a USB version of the Zip drive. My previous Mac didn't have an Ethernet connection, and the Cube didn't have a port for LocalTalk, so for me at least this sneakernet was the easiest way to migrate data across--and since my old Power Mac only had a 1.2GB hard disk, it wasn't an onerous task.
Even once hard disks became so big in relation to the capacity of the original hundred-meg Zip disk, I still used them to store specific projects. There was and is something satisfying about compartmentalizing jobs, and there's something far more conceptually agreeable about taking a case down from a shelf, slotting a disk into a drive and so being prompted mentally to change gears into a particular work mode than there is about just double-clicking a folder on a multi-terabyte external RAID or NAS.
The original Zip disk was, of course, superseded by a 250MB model and then a 750MB variant, and God bless them, Iomega doggedly tried to reinvent the basic concept of the removable-disk--based storage system for years to come, introducing the Jaz drive (debuting at 1GB) and then the initially 35GB Rev drive, which eventually reached 120GB.
It was all for naught, though. Despite there being tremendous practical value to cheap-ish, removable storage--not least for backup, where a reliable, rotating offsite storage solution remains the gold standard--and despite nostalgic old buggers like me still finding the clunk-pause-whir-thut-thut-thut of a disk-based system inherently pleasing, both the nascent cloud and the move to USB-connected, flash-based storage meant the Zip disk's days were numbered.
Before the final year of my graphic design degree I asked for, and was given, an 8MB USB drive for Christmas, one of the original Disgo models. Where a Zip disk required a special drive, thick, unwieldy cables, drivers and a power supply, the Disgo just plugged straight into a USB port. I remember in an emergency during my degree show loading the Zip drive's drivers onto the Disgo so I could mount a Zip disk on an iMac in the exhibition space and transfer some files. 8MB was small even then, but by the same token even then it was clear that this fast, solid-state, driverless and bus-powered external disk was the future. (Now it too is destined for the past as we become more reliant on the cloud, but Think Retro might survive for long enough for USB flash drives themselves to become a topic!)
Let's round things off with some math, because who doesn't love math? And don't worry; the good news is that we're going to let computational engine WolframAlpha do the heavy lifting for us. If you go to wolframalpha.com you can take advantage of its natural language processing to work out how tall would be the stack of Zip disks you'd need to store the data on your biggest hard disk. For me, that's my Drobo, which currently has a capacity of 5.42TB. So all I do is enter "(5.42TB/100MB)*6mm"--since Zip disks are 6mm thick and hold 100MB of data--and I see in the "Comparisons as height" section that the resulting tower of Zip disks would be a little taller than the Eiffel Tower in Paris.
To be sure, I'm fudging things a little here. Files larger than a hundred megs--which aren't uncommon these days--would need to be split across multiple disks, we're not taking into account the overhead of directory structures, and I haven't bothered to dial in whether these capacities are in base--10 or base--2 (though WolframAlpha, to its credit, can accommodate this). You should be careful to use uppercase to denote terabyte (TB) and megabyte (MB), too, as if you use Tb and Mb, the calculations will be off by at least one order of magnitude.
Regardless, this gives you some sense of how huge today's hard disks are, and of what--if you'll excuse the pun--a towering achievement that explosive growth in capacity really is.
(I haven't mentioned the Zip disks' Click of Death, partly because I never experienced it myself, but mostly because I don't want to rob you of the catharsis of bitching about it in the Comments. I cede the floor!)