Have you been using the drive as 360 storage or to play media files from when plugged into the console?
If the former, are you leaving it permanently plugged in to the console? Some have reported that in doing so the drive is not recognised. Try removing the drive, restarting the console and then putting it in again, it will then hopefully be recognised again.
If the latter, have you recently formatted the drive to NTFS? The 360 only recognises FAT32 and HFS+, so you will need to go back to one of those in order to get the drive to be recognised again for media playback.
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Hey I have a Lexar 16GB and even though its properly configured and passed all tests. It shows up as unconfigured on the system startup. Unplugging and replugging makes it work again, even though its very annoying to do everytime.