I had a problem with my ZX Spectrum microdrive - when ever I put a cartridge in it and typed 'CAT 1' (to get a tape listing) it would spin for a couple of seconds and then say 'Microdrive not present'.  I bought another Interface 1 from ebay, as I read that a blown ULA could cause this problem, but that didn't help - so I had two interface ones, two microdrives, and four cartridges, all with the same problem.

I solved the problem by:

1) Cleaning the tape heads with rubbing alcohol

2) Replacing the felt pads in the microdrive.  I think this was the think that actually solved it.  I took the felt off of a normal cassette tape, cut it to size, and then glued it into place.  To get the pad into place a carefully slackened the tape with a cotton bud, and then wound it back into place my moving the tape it the side of the cartridge to wind it back in.  This had a 100% success rate!