I've had two cheapie flash drive from Microcenter, and they booth out-of-the-blue became unformatted. When you insert then in the USB drive, windows says "The drive is not formatted. Do you want to format it now?" And then it can't actually format it.
Nothing happened to them physically. For one, it 'died' during a reset, and the other, it was normal and then it was dead once when I inserted it, IIRC.
What's the deal? Are these just cheap, crappy drives, or what?