It really depends on how important the data in your runtime is to you. For really important data one surely wants to use a backup medium that won't get corrupted when in use, so a hard drive isn't a good idea for that. (Like a virus/power surge wiping/killing your backup HD at the same time it wipes/kills your other HDs...) So for convenience, use a HD (that you pull all plugs when not in use for backup). For more safety use two HDs. (Alternating backups between the drives). And then also back up to DVD from time to time. And if it's really important then store the DVDs at another location ;-)