I have considered a backup solution of this type before.  The thing that makes it a bit risky is the fact that your backup data is still connected to your server. If you were to contract a virus, the virus has access to your live production data files and your backup data files.  Having more then one harddrive to connect to the server lowers the risk a bit. Having four drives and using a different one for each week of the month maybe.  This plan too falls apart if you don't discover the virus in a relatively short amount of time.

Thanx,