Just for your information; unlike told in the tutorial, 2GB usb drive seems to be not enough for BackTrack 5R1. I tried to install the 32bit and 64bit GNOME versions on it but UNetBootIn stopped at about 50% and told that the stick was out of space. Installation onto a 8GB stick went well so it's not a bug with Unetbootin or anything.
Of course, if you really wanted to have some fun, go to Wal-Mart late at night and ask the greeter if they could help you find trashbags, roll of carpet, rope, quicklime, clorox and a shovel. See if they give you any strange looks. --Streaker69
Same question as Barry, which tutorial?
The official wiki talks about 4Gb for LiveUSB and 8Gb for Persistent USB install.
This is the thing, when looking for a tutorial/documentation, try to look first in the official website of the tool you're using. It should be the best place to learn how to use a tool (who better to teach you how to use it than the author itself?). For example, I learned a LOT about wireless networks just by following aircrack-ng tutorials, where everything is very well documented and explained. The same can't be said about other tutorials in the wild (although some of them are pretty good and explain everything in a easy way to understand).
THIS ONE - which is still in the site index nine months later and states "(Minimum USB Drive capacity 2 GB)"
Glad I found this thread before wasting the time on an old leftover 2GB stick!