I completely rewrote the question since this turned out to be a storage spaces question.
Long problem short: 2x3TB disks -> fill upp space -> add one More 3TB disk.
Now Windows says disk is full, but the disk pool has lots Of free space.
How do I solve this?
Sorry, I was in a rush and left out lots of info:
1) The disk was 2TB from the beginning, I overprovisioned(?) the disks, i.e I made the disk 2TB (parity) and 8TB (normal) from start even tho I didnt have enough pysical space under them (onlt 2*3TB).
2) I added another 3TB so I have a total of 9TB physical disks.
3) Continue to write to the "normal" 8TB disk is no problem
4) writing to the 2TB parity drive, which has 1,57TB free space gives me this: http://i.imgur.com/floZle6.png
5) Storage space manager: http://i.imgur.com/e650eJ8.png
I think that this is the "problem": Both physical drive 1+2 are full. Writing a file to the "simple" drive is no problem, since that file can be placed on physical drive 3 alone. But when I want to write a file to the parity drive, that file has to be placed on at least 2 physical drives, and since both physical drive 1+2 are filled up, thats a no go.
If I delete for example a 2 gb file from the "simple" drive thats older than when I added the 3rd drive, I can then write 2 GB to the parity drive. I guess thats because I now have freed up 2GB from either physical drive 1 or 2.
I need the drives to do "rebalancing", i.e move files from disk 1 and/or 2, to disk 3, so that the parity drives files can be split amongst at least 2 drives.