It's the bit in my post above that I crossed out. I'm glad you either missed or ignored that comment and used "mount -o loop" instead!

When you're copying to your partition, the /dev directory should be empty in your mounted Debian image file. If it isn't, you need to either reboot or issue the closechroot command (as root) to make sure everything's unmounted... then you can go ahead and mount the image file. The /dev directory should be empty now.