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/