Well it cannot write to your storage area because permissions are wrong there. It must be writable for the ‘backup’ user. You should have seen errors at install time (see /boot/first-boot.log).
If so, reinstall Amanda.
Eventually (if you mounted /mnt off some standard UNIX machine) you forgot to add the no_root_squash option there.
You didn’t mount a CIFS share, did you ? It is not supposed to work and you should know if you read the manual. Let alone it’s nonsense as Windows machines usually don’t run 24x7.
I have some issues with nfs, therefore I try it with cif! This share on qnap is also used by windows application and I activate samba v3. when I try to mount the share with nfs I receive a time out.
I think with cif you have more option, regarding permissions. I´m not so familiar with unix systems…
No, you might not have read about or tried, but in fact Linux is more flexible there.
You can also export a disk as both, CIFS and NFS.
Either way be aware that to use CIFS is not supported for Amanda storage for good reason as is explicitly stated in the README.
Good luck if you want to keep that setup but please don’t ask for help again here if it does not.