Almost twenty-five years ago, floppy drives were far from the only removable storage available (and don't even think about hard drives, which were too expensive for home computers for many years to come). While many popular home machines used cassette tapes to store data—including the Radio Shack TRS-80 series that I first cut my teeth on—even those were too bulky to include in some of the first portable computers. Epson's HX-20 used a micro-cassette drive instead, which allowed it to store not only data or audio, but hybrid data/audio combinations that could trigger sound clips recorded to the tape to add audio to a program, such as playing an alarm bell (at least I think it could; its successor the PX-8 can).
When first announced in 1981 it was a revolution in its class—a four-pound machine whose closest competitor was a 24-pound Osborne 1. Powered by a Motorola-esque Hitachi 6301 processor running a 0.614MHz with 15K of RAM and 32K of ROM, the HX-20 was quickly adopted by the newly-minted mobile set, including journalists, who found the machine's 4-line, 20-character screen ample for writing copy on the go using the burnt-in copy of WordStar. Its 50-hour battery life from rechargeable NiCad batteries didn't hurt, either.
Used HX-20 can be found occasionally online for way less than $100 (I'm currently bidding on one, so don't snipe me, you bastards) and Epson, to their credit, still maintains a support page for the HX-20, with user manuals and troubleshooting tips in PDF. (Thanks, Grobaty!)
HX-20 Documents and Manuals [Epson]