I don't have a 7D yet, but I just updated firmware on my 550D. Despite meticulously following directions over and over I kept running into problems. Until I switched SD cards
my Duracell ProLine 64GB SDXC Class 10 Card would not let me upgrade... but my Panasonic 8GB Class 4 SDHC would.

So if someone runs into this maybe I can save you the frustration I was having the last hr.