Sharkbite, please learn from my mistake...for years I backed up projects as insurance against what I thought were the most probable disaster scenarios: disk failure and user-error. When disaster really did strike, though, it wasn't either of those things but something unexpected: theft. The thieves took 12 years of projects
and their backups.
It could have been fire or a lightning strike, too, with the same result.
The only way to guard against all threats is a removable drive that you routinely disconnect and stash somewhere away from the computer. If your studio is in the garage, keep the backups in the house. If your studio is in the house, keep them in the garage. If neither is an option, use the glove box in your car as your offsite storage facility.
While an extra internal drive offers speed and convenience (handy if you back up frequently), I would urge you to consider an external drive (or high-capacity USB stick) as the ultimate fallback. There's no reason you can't have both.