I have a Segate 500GB internal hdd (ST3500820AS), bought it about 6 months ago. It was working fine but now i am facing this problem. The system is running with vista installed on another 250GB hdd. System start's up but once loading bar is done, it goes into a long lengthy wait, a blank screen for about 20 mins follows and then loads that vista welcome screen. Even when explorer is loaded, i can see all the partitions, but can't access partitions under 500GB hdd. But all partitions under 250GB hdd work fine. I get the error "<drive> is not accessible because of an I/O device error" for partitions under 500GB hdd. I at first tought the hdd was dead as the system would start normally, at normal pace without the 500GB hdd. But later discovered that the hdd is still detected in the bios.
I guess this could a partition corruption 'cause when i tried to reinstall vista assuming vista must have been corrupted, it showed me 465GB as unpartitioned space, which is the actual capacity of the 500GB hdd, when i actually had three partition of "37GB, 284GB and 144GB". It tried some recovery softwares, which either crashed or failed to load the partitions.