You need NAS. Before I get flames, I'm talking about Network Attached Storage. Keeping your backup on your primary drive is a bad idea, IMHO of course. If anything happens to the drive, you lose all your backups.
If I may ask, why are you keeping so many backups of Windows? You can always re-install Windows if something goes wrong but it's very hard to recover data. I usually just backup the important files and don't backup the whole OS. Just my .02 cents.