The HDD needs to be formatted to FAT32. I know I just backed up and replaced my HDD in my PS3 recently.
I don't think you can actually back up your save file at this point but you can back up the entire PS3 and not lose anything.
There is an App by HP you should be able to find(sorry I will see if I can find out where I got it, I'm on my laptop)
Win7 won't let you format using FAT32 and I also know Windows won't do large drives anyway
Will Edit this post when I find a link to that App.