wbadmin/backup cannot include Disk

Hi,

I am administrating a windows server 2012 and I try to perform a scheduled backup for that machine.
So when I just include C: and (Metal Recovery stuff) in the backup plan everything works out as expected.HOWEVER, as soon as I include another drive D: it quits before it copies anything. For testing purposes, I created an empty folder "Test" and only included D:/Test and it still doesn't work. I happily provide further information, I just don't have any idea how to tackle the problem.

Thanks in advance,

Jannis