BIOS detects secondary SATA HD, but XP doesn't!

Hello board, I seem to be in deep trouble. This is about my content repository PC that I only used to store all my important files I've been working on since last 5 years.

Config of this PC :
OS : XP Home
HD1 : IDE 40GB (Primary) - 3 partitions FAT32
HD2 : SATA 250 GB - 4 partitions - NTFS [in trouble right now]
HD3 : SATA 1 TB - 4 partitions - NTFS
RAM : 256 MBs SDRAM.

I have most of my highly critical stuff on this 250 GB Sata drive. Installed my new 1 TB drive some 3 months back and had to backup all my data from my 250 GB to 1 TB HD. Didn't get time to do that. Started with that 2 days back, and all was going fine, copied some 20 GB of data, and suddenly there was an error while reading from source disk. The system was becoming a bit jammed and overloaded for unknown reasons, and I thought to restart the system. After I restarted, I get to see that though it can detect all partitions of 1 TB, all partitions of my 250 GB drive becomes non-detected. I restarted several times, and noted that while booting, it was taking a bit more time, almost 1.5 minutes at the "Detecting IDE Drives..." stage to detect my SATA drives now, and both the drives get detected in that process... After the point that says "Verifying DMI Pool..." and then the windows load, later, the 250 GB drive is not detected by the loaded XP.

Now, neither the drive partitions are visible in My Computer, nor I can access them through command prompt. I'm deeply worried because all my data still resides on that hd and that's killing me every moment. Please help, I've no words to say other than that :(

I haven't even used this PC too much... I only boot it once in 7 to 15 days, just for a few hours, transfer my stuff from my other PCs to this PC, and then shut it down. So the disks haven't even been roughly used as compared to my other PCs :(

I showed the system to my hardware expert, he plugged the drive on other PCs, and it wasn't getting detected at the boot level there. He then suspects that the green PCB on top of HD should have some problem, and he has told me to wait for some days till he arranges that pcb and puts that on the HD and test again.

I seem to be in deep waters. I'm not an hardware expert and I don't know how partitions and all work, but even being a layman on this front, I do suspect if its the matter of partition table getting corrupt or something related to that. Please help and guide me in the right direction. With all my gratitude and thanks in advance.
 
Update to my previous condition...

An update,

I ran AVG antivirus on that software while having the 250 GB drive plugged in. It scanned the primary disk and the 1TB drive, and then asked for reboot, I allowed, and on rebooting, it selected the 1st partition of 250GB drive for scandisk! It scanned it, marked several segments as bad, and then met with "an unexpected error" and terminated the scandisk... Booted into win xp, and here comes my 250 GB alive... I then checked the Event Logger, and found lots of system event alert logs specifying the issue related to bad sectors!

Anyway, as all 4 partitions were detected now, I jumped in to backup files to my 1TB drive... but as soon as I picked one software , did COPY and PASTE on my 1 TB drive, it said ... can't ready from source drive and the storage medium might have some errors... And with that, all four partitions of my 250 GB drive again become inaccessible!

Please help!
What should I do now?
Thanks a lot in advance.
 
Mount the Drive

You may have to manually mount the drive in windows or at least you may have to download the drivers from the manufacturer for the disk, however this would be unlikely since most hard drives are plug and play.
 
Back