After creating a bootable USB image for BackTrack 5 R3 with Unetbootin you may encounter an error:

Remove disks or other media
Press any key to restart

This means you have incorrect image on USB drive.

Simple solution is to use a different ISO to USB utility:

You can download it from following URL:
http://www.pendrivelinux.com/universal-usb-installer-easy-as-1-2-3/