If you have a 'backup' on the same PC, you don't have a viable backup at all. If the PC's HDD fails (and mine did, about 6 months from new) you're completely f***ed, you've lost the lot.

I have an external USB HDD that I back up (and, equally importantly, verify the backed-up data) all my data on weekly. Makes sense.

When I was working we had a company rule that, in a network of 50 or so PCs, no data was stored on local (i.e. PC) HDDs - all saved files had to be saved on the network servers, which were backed-up every night.