ArtooDetoo:
Are you talking about the internal SD or the external SD? Backing up to somewhere you're about to flash doesn't seem very safe. SIM is fine because it won't be touched, nor will the external SD. Especially since you can remove them... :-)
I thought it was the external SD card (well there were no other choices given) but clearly it wasn't since the card had no data on it!
You live and learn I guess.