Update: TrueCrypt 7 and later will allow you to bypass this check.
TrueCrypt, the freeware open source disk encryption product, allows you to encrypt your whole hard drive. TrueCrypt calls this “System Encryption”. Another name for it is Full Disk Encryption. Just like Microsoft Bitlocker.
When you encrypt your system drive, TrueCrypt prompts you to create a Rescue Disk. Should the TrueCrypt Boot Loader be corrupted, the Rescue Disk will help you fix that problem.
But TrueCrypt requires a CD/DVD drive to create the Rescue Disk, and if you don’t have one, it won’t let you continue: