Dreaded "Cannot Configure Backup Schedule" on SBS 2011
http://social.technet.microsoft.com/Forums/en-US/smallbusinessserver/thread/6f4bb5f5-d411-43fb-a365-170048bec70f/
I found the answer below:-
http://www.deltech-cs.com/blog/archives/add-multiple-backup-destinations-to-windows-sbs-2011-when-not-all-backup-devices-are-available-onsite/
If you want to add multiple backup destinations, it is also simple as long as you have all backup destinations connected to the server. There are certain situations when this may not be possible or ideal. You may not be able to connect more than one backup device to the server at the same time, some backup devices may not be available (they are stored off site), or you simply may not want plug in every single backup device just to add another backup destination.
Whatever the case may be, if you try to add a backup destination when not all the backup devices are connected to the server, you will get an error message, and you will not be able to complete the procedure. This behavior is by design. When the wizard completes, it verifies the currently listed backup disks. If any of the disks are missing, you will receive an error message, and the operation will not complete.
To add multiple disks when not all disks are available or connected to the server you will have to use the command line. Plug the hard-drive in to the server. Log in to the server using an account with administrator privileges. Click on the start menu, click on All Programs, click on accessories, right click “Command Prompt” and click “Run as Administrator.” This will open an elevated command prompt. On this command prompt type the following commands:
wbadmin get disks <—- use this command to determine the Disk Identifier of the new disk. Examine the output and locate the disk that will be added to the backup schedule. Make note of the disk identifier.
wbadmin enable backup -addtarget:Disk Identifier – make sure you include the brackets <— This command will add the new disk to the scheduled backup.
Once you run the last command you will receive a series of prompts. Type Y to accept all of them, and you are done.
Also here is Microsoft's description of issue and 3 solution options
http://support.microsoft.com/kb/2009365