DVR External hard Drive - Movable?

We have two DVR boxes in our house. We just got notified of the upgrade in Omha and our boxes were remotely upgraded (our settings are now different). Now that the esata port has been activated, can we attach a drive to the upstairs box and still be able to use it with the downstairs box? Or does it need to be reformatted?

I'm assuming that shows recorded on one box cannot be seen by the other box even if the drive can be moved.