I'm attempting to install Backtrack 4 R2 for the first time on my USB drive with encryption following the tutorial in the "How To" section.

This is my input:
root@bt:~# cryptsetup -y --cipher aes-xts-plain --key-size 512 luksFormat /dev/sdb5

And I receive this output:
This will overwrite data on /dev/sdb5 irrevocably.

Are you sure? (Type uppercase yes): YES
Command failed: Can not access device

How can I allow it to access the device? Or what have I done wrong?