Hello!
Using the tool Repair Filesystem helped only partially. The problem was complex - from time to time system freezes and only hard reset returns it to work. In the log was something like a "bad stripe". Found a possible complex problem with Adaptec / ICP Storage Manager

http://ask.adaptec.com/scripts/adapt...ted=1172018702

and Seagate Barracuda

http://ask.adaptec.com/scripts/adapt...i=&p_topview=1.

Had to update the HDD firmware to recommended and build the raid array again (backup your data!!!). 5 months NAS working as a clock . Maybe my experience will help for someone. Thread can be closed.
And thanks for the help!!!