My suspicion is it's related to the USB drive itself, not a bug in LMCE/Linux. I used to see this on occasion with a WD mybook essentials (or something like that). When that drive started to die I replaced it with a 1TB Fantom Drive, and have since added a second 1TB Fantom Drive. After swapping out the original WD I have not had any issues. Well, at first I left the FD drive as NTFS (or was it Fat32?) like it came, but that caused all kinds of problems. After I reformatted it to ext3 I have not had a single issue I can remember with a drive not mounting when needed.