I have never stored a CD in a place where it can freeze, but I always imagined that it was kind of like a tape cassette, and would just withstand the cold. But now I have been told that this is not the case, that the silvery surface can get cracked and damaged by frost.
I have no idea if this is also true for hard drives, but I am thinking that it is not worth taking the chance. Anyway, I think that the best way to back up a computer is to just buy a removable hard drive, plug it in to a USB port, and save your important files.

Then, and this is critical - store it in a different house than the one where you live!