I encrypted a flashdrive using TrueCrypt, but now I've changed my mind and I want to format it as an unencrypted drive. When I plug it in it is no longer appears on the launcher or in the Nautilus sidebar (until I mount it as an encrypted drive via TrueCrypt, which appears in the Nautilus sidebar, but even then the flash drive itself is not recognised).
How can I access/mount it in order to format it?
Disk Utility
, what do you get? You really should see it listed, mounted or not. Can you please try that and edit the question?