You can still burn from an external hard drive. There is no reason why it wouldn't work properly, unless you hooked up something incorrectly.
When it comes time to burn, select Burn Image in Nero (or whichever program you use), and navigate to where the IMG is located on your external hard drive. If you have burn images before, then you'll know what I'm talking about. The external hard drive will appear just as an internal hard drive would. It will have a name associated with it and everything (i.e. D: or E:). One note, which I just remembered, Nero won't burn IMG's: only NRI, ISO, and BIN/CUE. So, either convert the image to one of the formats compatible with Nero or use another burning program.
No, I like women.