Recently I've also looked inside microdrive drives of Sinclair QL and inside microdrive cartridges for it.
The tape looked like a compact cassette tape cut longitudinally to a half width (which seems to be true -- microdrive tape has width of 1.9mm, while compact cassette tape is 3.85mm wide). The head also looks like a common head in compact cassette players or recorders.
The schematics reveals that the single microdrive head has also erase coil, while the compact cassette recorders usually use separate erase head.
The question, therefore: Were the compact cassette technologies really used in microdrives, modified or not? If yes, what modifications did Sinclair implement to it?