You might want to use MD5 to check downloaded ISO, then re-do the USB install with unebootin.
Some problems have been arisen with USB 3.0 acconrding to some posts, so you might want to use a USB 2.0 unit, just in case.
Also is highly recommended to burn DVD and use it.
Please, refer to the wiki if any doubt as far as "USB Installation": http://www.backtrack-linux.org/wiki/index.php/Main_Page
Section : Getting Started" ----> "Installing Backtrack"
Hope it helps.