Just to clarify,, I think you are talking about running Ubuntu from an
install on a SD card, whereas the OP was planning to backup his existing
install to the SD card, then do a dual boot on his main drive. Correct me
if I'm wrong, I just didn't want the OP thinking there was something wrong
with backing up on the SD.

Then again, I may not be clear on what timmytheimpaler is doing, either! :)