Setting the scene
One of the biggest drawbacks of many of the 8-bit computers on the 1980s was long-term storage. Audio tapes were the norm, and for good reason; they were cheap, easy to use and the computer-illiterate (or often computer-fearing) public of the time were comfortable with something they could already use. There were, however, big disadvantages of using audio tapes; they were often unreliable (although the Spectrum's format seemed better than other in this department) and they were enraginginly s-l-o-w. The linear nature of the format was also irritating. If you had more than one file on a tape, you had to keep painstaking records of where each file was using the tape-counter (which changed unpredictably between different players) or spend long periods of time fast-forwarding and rewinding in the hope you'll find the right one. Some users (including me) evolved the uncanny skill of being able to tell where tapes had often been stopped between files by looking at the tiny differences in shade in the reel of tape inside the cassette.
The stage was set for the introduction of the floppy disk to the home computer user. In those days, of course, floppy disks were actually floppy - they were big 5 1/4 inch things encased in card, not plastic - and the drives were huge. They certainly didn't seem to fit the Sinclair way of doing things and so, as was his way, Uncle Clive went in his own direction. He introduced the ZX Microdrive.
The microdrive is a permanent storage solution for the ZX Spectrum which are somewhere between a tape and a disk. It uses hard-shelled cartridges instead of floppy disks which contain a loop of magnetic tape. A motor in the drive unit feeds the tape (in one direction only) past a read/write head at 30 inches a second (16 times as fast as a cassette recorder). The cartridges hold, usually, 80-100K but I've seen them go a bit over.
What's it like?
Firstly, they are small; much smaller than the other storage devices of the day and those to come. At 90x85x40mm, a microdrive will sit in the palm of your hand. The cartridges are tiny, and are the best sized media I've ever used. They're as small as you can get without being annoyingly small, and they're protected in a nice plastic case. The drives and cartridges are black (like all Sinclair hardware), and the drives have the Spectrum colourful flash across the
How does one use such a cool little black box?
Well first off you need a ZX Spectrum. Then, you need an Interface One plugged into it. Then you plug your first microdrive into the side of the Interface One using the lead provided. Got more than one? The double-ended plugs provided let you daisy chain microdrives together -- you can have up to eight on a single spectrum. There's also a plastic plate you can use to screw chained drives together and so make it all a bit more stable.
So, how did it do?
Not well. They were released late; coming out about eighteen months after they were first announced. They were also quite expensive; you had to buy an Interface One for £30 (for which you did also get an RS232 port and Sinclair network ports), and the drive for £50. And they were still slower and smaller (storage-wise) than other media. By then, third-party companies had already launched their own solutions and the Spectrum was doomed to not have a standard storage medium (other than tape) until the Spectrum +3.
Later, microdrives were fitted to the QL. They were slightly faster and could fit a bit more on each cartridge, but by then the microdrive's image was already far behind that of its competitors and their inclusion on Sir Clive's all-new machine probably hurt its image rather than improved theirs.
Personal note: I couldn't afford one of these myself when I was speccy-obsessed youngster, but came across one later as an adult and couldn't resist. It has to be said, they are rather cool pieces of kit and fit much better into Sinclair BASIC than other solutions I've seen. Don't believe what people say about their reliability, either. The one I got had a box of carts that hadn't been used for ten years and still worked fine - I'd like to see you do that with a HD floppy. If, like me, you're still of a bit speccy fan in the 21st Century I really suggest getting hold of one (or eight) if you can.