See for a possible bug with

Either using a live CD or a live USB and then using the command, sometimes the long process of applying a persistent BT4 Pre-Final is successful EXCEPT for the grub installation.

You can tell if this is the case if you attempt to boot into the newly minted persistent usb and just get a blinking cursor after booting.

If so, do the following:

...Before logging out of BT4, pull up an xterminal and type in:
...mkdir /mnt/sdb1
...mount /dev/sd1 /mnt/sdb1
...grub-install --no-floppy --root-directory=/mnt/sdb1 /dev/sdb


...nano /mnt/sdb1/boot/grub/menu.lst

and adjust menu.lst


...unmount /mnt/sdb1/

Or, more simply forget and use the BackTrack USB Install video here:

Backtrack 4 USB Install - Offensive Security

Hopefully, this bug will be fixed in the BT4 Final.