you can tar directly to any raw partition on a harddisk (just like done on floppies and tapes) but please note that in the event of a chrash you backup might have problems also... so its only usable for (short term) backup purposes needed for reinstallations...
tar cvzf /dev/hdc [files...]
the complete /dev/hdc partition would be your tar(gz)file
ps. don't try to mount the partition, its not mountable, restore with tar xvzf /dev/hdc