Are you doing this?
It is a great way to keep your stuff backed up without thinking about it. Most SATA controllers have RAID these days so you just install another drive about the same as the one you want backed up and it "mirrors" the original all the time.
Drives are cheap enough that this is not really that expensive. If a drive fails you get a message, swap out the bad drive and the system re-syncs to the new drive. Nothing is lost. You can run single while you are getting your new drive but be aware you are not protected.
