Always do a backup first

Of course there is always a risk that something could go wrong, even from causes outside of the control of TrueCrypt, the computer or you. for example a power failure at a bad moment could corrupt the data. Especially during the initial encryption of the disk, which does take some time to do, it is possible that something like a power failure could happen. Therefore I highly recommend that you backup all important data to prevent problems. Other than that, the risk that the disk would be messed up is minimal. KRB