Inserting 3rd drive into RN 214

I have a ReadyNAS 214 with two 4TB HGST disks in RAID 1. I bought the RN to replace a failed Seagate single-disk consumer NAS device, which is currently sitting down at the local Frys service center. They're trying to recover the data for me, but are having "issues" with their recovery software. Honestly I think it's just incompetence, so I was contemplating heading down there and picking up the Seagate, bringing it home, pulling out the actual drive, and inserting it into slot 3 of the RN to see if I could pull the data myself.

Is this possible? What will the RN's behavior be when I insert the drive? I'm not a server/RAID guy so I don't understand all of the nuances, and my fear is that the drive will somehow automatically be wiped and added to the RAID array, though if I understand the basics that's not possible since the drive does not match the drives already in the array. It's a 2TB drive. Not sure what manufacturer.

If I add it, is it possible to configure it to be a stand-alone drive and allow access that way without wiping and reconfiguring it?

Thanks for any assistance/advice.