I'd be more inclined to store the most vital data on a thumb drive (memory stick) and put that in a freezer, since it has no moving parts. I'd be leery off keeping a hard drive frozen, and maybe having it bumped around and the case getting cracked when shuffling frozen food. I'd also think there'd be some risk when you took it out to do another backup, with the temp rise while you backed it up at room temp followed by a drop when you put it back in. And what happens when you put unfrozen meat next to it in the freezer? It takes awhile for it to freeze, so would it partially thaw and then refreeze the drive? What about condensation, and moisture damage if ice forms around it?
Maybe a good strategy would be to keep a thumb drive with the most critical data in the freezer (in a sealed ziploc freezer bag), another in a fire proof safe, and a regular external drive more accessible to do full backups more often.