I wouldn't consider a drive attached to your computer 24/7 a backup, more like a simple copy of your data, they're very different things. It's at risk of being accidently deleted or open to attack at the same time the computer it's connected to is, so it isn't safe to have it connected at all time.
You can use cron to mount a drive via uuid to a mount point then make an rsync cron job to backup incrementally. uuid shouldn't change after partitioning the drive so I'd imagine you could label them and swap them out every so often. But I haven't tried anything like that.