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:

Often the laptops I carry don’t have CD/DVD drives, which means the laptop fails the Rescue Disk creation check, as it cannot burn the ISO file to disk.
The way around that check?  Start the TrueCrypt Format program with a /noisocheck switch.  TrueCrypt will still create the ISO file for the Rescue Disk, but it won’t check that you have burnt it to disk.

This is handy to know if you are deploying TrueCrypt to lots of computers at once.  You would store each computer’s Rescue Disk in a central location, and burn them to CD as needed.
On a final note, TrueCrypt warns you (see above) that each Rescue Disk is unique to each computer.  In other words, you can’t create one disk and use it on multiple computers.  I had not realised that before.

Related posts:

You don’t need to bypass the TrueCrypt Rescue Disk check requirement now. TrueCrypt – not as secure as I thought. Why I wouldn’t swap hard drives on a laptop.

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