Found that I couldn't get to my D disc...
Went to ServerManager and Volumes, Discs and can see that the D disc (which contains 2 640Gb and 1 1Tb discs) of total 1Tb is Offline.
There seems to be 173Gb free space left, it is 91.6% full.
Tried to set it to Online, but it goes back to Offline directly.
I have 1 extra disc available, so I want to add this disc to this Storagespace, but it seems that I can only do this when D is Online... Catch 22
I have a Swedish version of the OS, so I might be using incorrect nomenclature...
I have an E disc aswell, containing all my movies (5,45Tb), and that disc is unaffected.
If I just can get the D disc up and running I can move files to the E disc...