I have a collection of USB drives in my laptop bag, as I suppose we all do, and recently I wanted to use one to give someone some files. This wasn't a temporary hand-it-over-they-copy-get-it-back scenario, so I wanted a pretty much empty drive. I found one with only 3 files on it, but the bad news was one of the files was called ReadyBoost.sfcache. That means at some point I had put the drive into my machine and told Windows 7 it could use it to speed up my machine. That's a good thing, and I'm a ReadyBoost fan and all that, but I wanted to use the drive for files and it was pretty much all filled with the cache file.
I tried deleting it, but no go. I tried ejecting the drive and putting it in again so I could say no if I was asked about using it, but I should have realized I hadn't been asked earlier when I put it in so not surprisingly I wasn't asked this time either. A little searching revealed the trick. Put the drive in, open My Computer, right click the drive and choose Properties. Go to the ReadyBoost tab. Presto!
Check "Do not use this device", click OK and tada - the cache file disappears and I have my drive back.