I solved the problem!!!
It was something wrong with my DVD-drive from witch I tryed to update (or, may be, IDE cable or south bridge on motherboard or something else, I don't care).
I just unplugged the DOM from NAS and plugged it into my desktop PC. Then I boot up desktop PC from the update CD (same as before) and... "TA-DA!!!" - "Update successfull".
I returned the DOM back to NAS and now everything works perfectly.
Thanks to everyone, who tryed to help me!