I own a few ILO HD04 which are Liteon clones (80gb) and it 's known that the defrag function in the Liteon hdd models is bad, reported problem with the 5045, so most likely your 5055 is reacting the same way.
I had that experience of loosing some recordings once, the second time all my thumbnails were grey and didn’t play, I never defrag anymore. I also have a 740 and 760 (north american version of the 5055) and will never use the defrag on them.
I manage my files a little better now, I backup to disc any file I want to keep, and when I’m down to just those recordings that are backed up, I erase the hdd and reload the keepers and the hdd seems to performs very well afterwards, a little faster in responding to commands. Hope this helps.