install ubuntu 12.04 on an external hard drive

Hello,
I followed this tutorial ici for rpi, but I have a problem to select my hard drive sda3 in uEnv.txt file.

Any idea?

Re,

the ftab, need to change but i search on internet and i don’t find a reponse,

Thank’s you for the answers !

http://www.embeddedhobbyist.com/debian-tips/beaglebone-black/beaglebone-black-usb-boot/

You’re welcome.