I've had it happen to me as well. Bumped a USB drive to which I was copying files under OS X, and it disconnected. It left the drive in a really wierd state - I could see the files on the drive, but could not read or delete them. Using diskutil under OS X errored out when trying to format, and diskpart in Windows said the drive was read-only (but could not change that flag).

I ended up needing to use some low level tool from a USB drive manufacturer's website to get it working again.