It happened to me exactly the same: 80GB and worked OK for months until it locked up on searching disk screen. I tried everything you tried before.
At the end, I got the solution: to format the first partition: first of all I moved all the files to the other partition or to the PC so the partition didn't have any files, after that I formatted and now it works like the first day.
Hope this answer will help you.
David
February 2007