looks like it could also be caused from a corrupt header, in which case you have a chance to restore it from the embedded backup header. If the first solution does not work, attempt the following: truecrypt --restore-headers /path/to/the/usb/device you will be prompted and asked if you want to backup from a header backup you saved, or from the embedded header backup Truecrypt automatically added to the volume. Click on embedded and cross your fingers that it works I suppose. That is the Linux command anyway it might be different on the Mac version, try truecrypt --help if that command fails and see if there is a Mac equivalent.