The 211k has a menu option to disconnect the EHD. Use that option, it will reboot the 211k back to its old self with no HDD. Disconnect the EHD and plug the other one into the USB port and the 211k will recognize it as an EHD. If it has been on another 211 or 211k on your account it will reboot and be usable, if it is a new drive it will format and set it up.
Overall the swap of drives on the 211/211k is a little bit klunkier than on the 612/622/722/722k because of the reboots, but it works otherwise.
Reboots are required because the OS, the guide data and the recordings are all on the EHD.