I recently purchased a power supply and sata to usb connector. It did not cost much from ebay.
It allows me to connect a hard drive when there is no physical bay left.
It is ideal in this sort of scenario because you can connect it externally, add to the pool, then remove an internal drive from the pool. When all that is done, pop open the case take out the internal drive and add the one you had on the usb connector.
Start up and all is well!