Its more of a WAG
Here is a pic of my two set up. I use the 500gb for HD movies and the 1tb for tv shows. With everything on, depending on what I want to watch and what drive is connected, I just pull the usb cord and move it to the other drive, and it works very easily.
Here is the drive I bought
Ah, like the photo, easy to get now. Thanks also for differentiating the two units. So, sounds like you switch back and forth, depending on what is to be recorded? You have have two dvr units? Once made into routine, switching to the different dvr units is not much of a hassle. Very organized.