Here's my backup system (I confess I don't use often, as I should). My main machine has a pair of 320 GB Seagates - separate, not in any RAID. I bought a 500 GB Seagate and mounted it in an external case. From time to time I use Seagate's Disk Wizard software (actually, a version of Acronis True Image), first deleting all previous Partitions on the external 500 GB unit. Then I make a complete bootable CLONE of my C: drive to the external unit, specifying that it should use only maybe 250 to 300 GB of space for that. Then I use that utility again to make a second partition and copy to it all the data from my D: drive.
I think I've got things set so, if my internal C: drive fails, I can boot from the external unit's first Partition as the C: drive. If that does not work, the alternative plan is simply to move the 500 GB unit from the external case to inside the main computer case and plug it in to replace the original 320 GB C

rive. Should boot from there easily.
An external unit that can be turned off and disconnected is the best way to ensure that its contents are not seen and corrupted by any malware. If disconnected, it also is not going to be damaged by power surges etc.