Warning! Before you buy that external hard drive to use with Windows Server Backup on Server 2008 R2, be sure to read this post on Microsoft's website first. It was answered back in 2009 by Microsoft agents, but left open for more discussion.
I have yet to prove my theory, but what I understand is that most external hard drives above 1.5TB on the market today have a physical sector size of 4096. The problem with Windows Server Backup is that it assumes the physical sector size is 512. Supposedly, some of these newer external hard drives can emulate a 512 byte physical sector size. Microsoft released a patch that addresses most issues with Windows and larger sector-size hard disks. The catch, is that the hard disk has to emulate 512 byte size sectors to work with this patch.
My personal experience was with a Seagate FreeAgent GoFlex 3TB USB 3.0 on a USB 2.0 bus. Installing the patch and restarting the server did not fix the problem. The fsinfo command reported that the drive did not support the query for real size of the sectors.
Windows Server Backup requires its target disk to support 512 byte physical sector size, but in order for Windows Server Backup to confirm that, the disk must support the query.
I hope this helps.