Transfer Ext. hard drive from Series 3 to Roamio Plus

Installed a WD My Book AV DVR Expander (1TB, SATA) on my Series 3 with no problems about a month ago. Fast forward to today and I'd like to move the drive to my new Roamio Plus, not much caring if I keep any content recorded as such. After I plugged it in to the R+ I got a message that the drive was not compatible. AFAIK, it should be so I thought I had to "decouple" the drive in the Series 3 but the instructions were to just unplug and remove.



So, is the WD NOT compatible with the Roamio Plus? If it is, what do I need to do to get it to work?



Thanks in advance.