Google is your friend. it turned up this:
After speaking with a TASCAM service representative I was instructed how to perform a hard reset by holding down the "track edit" and "up arrow" keys simultaneously while powering up. From there it will re-initialize the disk and ask you if you're sure you want to. Answer "yes" using the "yes/enter" key, and the unit will re-boot itself and return to the welcome screen as loaded from the factory.