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The name Microdrive has been used to designate data storage devices. There are at least two (completely unrelated) devices bearing this name:
Sinclair's ZX Microdrive
The ZX Microdrive (photo (http://www.aceadvanced.org/computers/sinclair/optional/interface1microdrive.jpg)) was, typically of Sinclair Research, very cheap and technologically innovative but also rather limited. Connection of a ZX Microdrive to a ZX Spectrum was done via the special Interface 1 device, which had to be purchased separately. However, a total of eight Microdrive units could then be connected to the computer by daisy chaining one drive to the next.
ZX Microdrives used tiny postage-stamp sized storage cartridges containing an endless tape loop, like an 8-track cassette. The cartridges held around 85 KB. Unfortunately, the drives were somewhat unreliable, and the media were relatively expensive. Similar technology was used in other devices, such as the Rotronics Wafadrive, and was generically known as a "stringy floppy".
The Sinclair QL version of the Microdrive was hardware compatible with the Spectrum version, but used a different logical format. ZX Microdrives also appeared in QL derivatives such as ICL's One-Per-Desk and BT's Merlin Tonto.
IBM's hard disk Microdrive
 IBM 1 GB Microdrive The hard disk Microdrives were originally released by IBM in 1999 with a capacity of 340 MB, which was expanded to 4 GB by 2004. They weigh about 16 g (~.5 oz), with dimensions of 42.8×36.4×5 mm (1.7×1.4 ×.2 in).
These drives fit into any CF II slot; however, they take more power than flash memory and so may not work in some low-power devices (e.g., NEC HPCs). Being mechanical devices they are more sensitive to physical shock and temperature changes than flash memory, though in practice they are very robust.
In December 2002 Hitachi bought IBM's disk drive business, including the Microdrive brand.
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