Apparantly this can happen and there is FAQ item on this qnap site.
http://www.qnap.com/faq_detail.asp?faq_id=531
This did not work for me. Maybe because I did some other trying out before.
Anyway I tried took out one of the disks and put them hard drive dock. And I managed to mount it under linux as ext3 filesytem. Nice. So I managed to rescue my data and the reformat the drives.
Some more info on NFS. To mount from command line:
sudo mount QNAP_ADDRESS:/folder/already/setup/to/be/shared /home/username/folder/in/your/local/computer/home/music /home/poningru/music
Also something that could be useful from http://www.mpcclub.com/forum/showthread.php?t=13723
"user and grouid is a bit tricky to enable access in write from macosx/linux.... i mean that a new file should be created with one of the local user of QNAP then i specify it: (the write request anonymous is translated in user 500, group 500 (the first user, pass of yor QNAP.. see cat /etc/passwd and cat /etc/group then my exports file in /opt/etc is: /share/Public (rw,all_squash,anonuid=500,anongid=100)
"
