Hard disks can have errors that don't show in tests. A friend told me about one that passed every test he made until he found one which could determine an error. A customer of mine had instability problems which he believed to be hard disk related. I checked the disk more than once with more than one manufacturer's tests and fixed the issues. Within a week he'd come back with the same symptoms. After a couple of repeats I finally cloned the disk to a new one and lo and behold, the problems were gone! Must have been some kind of a bad joint somewhere.
My very first PC ever had a defective hard disk. I sent it to service 2 times and they returned it without finding any fault. Was driving me nuts. Finally it dawned to me that when they tested the computer, they used their own drive to boot it up. Once I convinced them to try it out using the original drive they got it to crash too.
So they replaced the hdd on warranty and didn't even ask me if I wanted to keep the data on the drives. Needless to say that was the first and the last time I ever used a PC service.
