Photo: Red Hill.
These were a wonderful idea. The Backpack hard drive was an external unit that plugged into the printer port. Inside the box was a stock-standard IDE drive from one of the major makers.
But why not just use USB? Because in those days there was no USB! USB didn't exist even in theory until 1996, didn't become at all common on new machinery until after 1998, and wasn't actually useful, trouble-free and practical before about 2000 or 2001. Even then, it wasn't suitable for a busy computer workshop because we couldn't use USB on older machines, and there are always plenty of those. (For years, technicians used to joke that USB stood for "Useless Serial Bus.")
Backpacks were around long before USB. Our first one had an 80MB Conner drive inside — that dates it to about 1992.
The Backpack device driver needed only a single line added to your CONFIG.SYS, consumed less than 5k of precious system memory, and it could be loaded high. In the days when we used Backpacks, they ran almost as fast as a standard 80MB or 540MB internal IDE drive (about 1MB per second transfer rate), and it took less than 30 seconds to fit one. Unlike Zip Drives, they were real hard drives with typical hard drive seek performance — about 12ms back then — and decent reliability too. You could even daisy-chain Backpack drives together and keep your printer too. The Red Hill workshop ran on Backpacks for years. We had four in all, each one bigger than the last, and until we got our first CD burner we used them all day, every day.
Microsolutions was still in business and still making Backpack drives until early in the new century. The company also made a wide range of external floppies, tape drives, and CD-ROM drives, which we used to use for old CD-less notebooks sometimes.