External Hard Drive compatible with Windows Server 2012 Essential R2 for taking back-up

WD External Drives External Drives for PC
2

You should be able to do the back up with any of the desktop or portable drives. Check the links below for more information. 

My Book

http://www.wdc.com/en/products/external/desktop/

My Passport

http://www.wdc.com/en/products/external/portable/