I downloaded an iso for BackTrack4, and then used unetbootin to put it on an external hard drive. I restarted my computer, and selected the USB drive from the boot menu, but I get a black screen and then "NTLDR is missing...press ctrl+alt+delete to restart". Am I missing something simple? Would it matter if there were other items on the drive?

I only have ubuntu 10.10 installed, but I recently changed from windows 7 and reformatted the drive. Any ideas or suggestions would be great. On the external drive i have an image of the old windows 7 drive.

You have selected the wrong boot drive. The only way ntldr would have shown up is from a version of windows on some drive or part of.