When mounting a TrueCrypt volume, the volume itself is not mounted as a physical drive and as such Drive Bender has no way of determine what physical hard drive the volume sits on. This causes issues with duplication placement and can cause duplication to fail.
FYI - If you are not using duplication, this is not an issue.
With the release of v188.8.131.52 we have added a new feature called "Emulate physical drive numbers". This feature allows Drive Bender to assign a physical drive number to volumes that do not have one, and as such allow TrueCrypt volumes to work correctly with Drive Bender.
Note in future releases this option will switch in automatically.