Has anyone been able to boot Backtrack 4 through usb using grub2's loopback loop iso mount feature? For me, it just endlessly loops trying to mount my local hard drives. Here's what I have in my grub.cfg:

menuentry "BackTrack 4 Final" {
loopback loop /boot/iso/bt4-final.iso
linux (loop)/boot/vmlinuz find_iso/filename=/boot/iso/bt4-final.iso boot=casper nopersistent vga=normal --
initrd (loop)/boot/initrd.gz
}