Not beeing able to backup partitions to USB disk connected to NAS

I have a Seagate GoFlex Home NAS drive, 1 TB, to which I have connected a USB drive of 2 TB that I whant to use for my partitions backup. The unit has a username to use that is the same as my username in Windows 7, but the password is different.

I have no trouble using the disk from Windows Explorer, nor is there any problem browsing to its share name as a NAS disk, when defineing the backup task.

But when the backup is going to be written, I always get a message :
"
Access to the file is denied. (0x40014)
function = "DeleteFileW",
filename = "\\?\UNC\192.168.1.7\Extern lagring\My_partitions.tib"
Tag = 0xF35F747B3B21F8C9
Åtkomst nekad (0xFFF0)
code = 80070005)
Tag = 0xBD28FDBD64EDB8BC.
"
This task was a new one, so the file aught to be the one that should be written.
But the task looks like it is trying to generate the file My_partitions(1).tib.
The file has not been created by this backup name before, and there is no file on the disk whith that name.

I have before used the same disk for ATIH 2012, connected directly to the computer with USB. I then used another backup name. I have since then reformated the disk.

I have thought that maybe the process did not have access rights to the disk it self, so I tried to "hard code" the username and password to access the NAS, but that did not help. So I tries to instead say that the job should be run as my windows 7 user (since I don't have any problem accessing and writing to the disk). Neither did that help.

The Nonstop Backup is working without problem to the built in NAS-disk. The not working backup task is trying to use the disk connected to that NAS-disk with USB.