If you want to use UNetbootin to install a dual boot BT4/XP (or along side with another Linux OS) and do it off an External USB drive (different from a pen drive)...

The External USB drive... must and I repeat must be FAT32 and not NFTS format.

Once I re-formatted my external HD to FAT32 and then set the BIOS to boot from the 2nd hard disk (USB drive), UNetbootin booted to the USB and I then was able to install BT4 to my HDD using the installation script, and then used the slider bar to adjust the partition sizes. All went without a hitch!

(I had only FLAC files on my USB drive and formatted to NFTS because it always stayed attached to my comp and NFTS seeks faster than FAT32).

Now I need to see about setting up a special partition that the separate installations of Ubuntu 9.04 and Backtrack 4 can both see, so I can swap files between both OS's. If anyone has some pointers about what I'm trying to accomplish, it would be a big help.

Many thanks to Lupin, Tape, Lincoln and Pureh@te for helping out.