Backup Exec 10 with QNAP

[Pik78]

Hi
We are using Backup Exec 10 to backup our files on QNAP.
On QNAP we have one Storage Pool with five RAID-1 Volumes (10 disks in total). The free amount of storage is almost 4 TB but the Storage Pool Shows that it is 100% occupied. Because of this, Backup Exec tells me that there is no space left on the QNAP and the backups fail.
Can anyone give me an Explanation why QNAP Show 100% occupation even there is 4 TB free space on it and how I can solve this issue?
Thanks for your help in advance.

[dolbyman]

well if there is no free space then it's not really BE's fault.

any chance if a volume management screenshot ? ..

and this 5xRaid1 sounds ...interesting ..why so complicated ?