sure, you can connect and format them. Make sure the bios is booting from the right disk (your own, not one of them) though or if they had a system installed it could boot from one of them.
You can also connect them when the PC is already running, a feature called hot-swap, by connecting first the data cable and then the power cable to the disk, and waiting a minute or so for it to get recognized. And maybe repeat that a few times if it doesn't at the first time.