Source HD fails during BkUp

[dnanian]

Yes, you can do the rotation with physical disks -- which gives you isolation from failures on the backup device -- or with partitions, which does not, but helps to protect from the kind of trouble you mention, or a more common case, where you delete a file you didn't realize you wanted to keep until you'd already overwritten your backup.

These things are rare, and it's basically up to you how much you want to isolate yourself from their potential effects.

But, really, drives are cheap these days, and your data is not, for the most part. $100 or so is a small price to pay for physical rotation, should that be something you want to do.