If you are getting a loud metal like - hitting - like sound from your HDD, then your HDD is dead or going to die. Theres very little you can do at this stage.
Like icecoolz says, connect it to someone else's PC first and see if its working there.
If the drive isnt being detected at all, then you need to adopt extreme measures:
1) Drop the drive from less than 1 feet high onto a flat surface. Drop it flat, and not on the sides. And dont throw it down, just let go of it , in such a way that it hits the table/ground flatly.
2) Then lightly hit the drives with your palm on both the sides(edges, not surfaces/faces).
3) Turn it upside down and connect it to the computer, and start the computer.
(Note, please do these steps only as a last resort- if you tried everything and the drive isnt detected, try the above 3 setps, in that particular order)
This should get your drive detecting. Now, as long as possible, do not try to shutdown (poweroff) your computer:
1) Download and run SpinRite 6, the best disk repair and recovery program. Note: do this on a friends computer; and you are going to need a blank floppy OR a blank CD to make a bootable Spinrite 6 disk
2) Restart your computer (remember, dont poweroff yet) with the boot cd/floppy of spinrite in. In spinrite's main menu, goto the second option, and then change the Level to Level 5. Then go back to the main menu and choose the first option. Then choose your partition(s)/disks and start the operation.
3) After SpinRite 6 is fully complete, then insert immediately the Windows XP Setup CD. Restart, Boot from the XP Setup CD and in the first screen, choose 'Recovery Console (R)'. In the recovery console, logon and type the following commands:
chkdsk C: /p /r
chkdsk D: /p /r
.
.
and so on (where C, D, etc are all your partitions)
After performing a full chkdsk, boot into windows. You should be able to boot into windows- IF you cant then use a bootable OS cd like ERD Commander 2003 or Knoppix 3.4 to boot from and recover your data.
Then you can do a full low-level format of your drive, followed by partitioning and reinstallation of windows.
If you CAN boot into windows, then please use this software: Active SMART to keep a watch on your drives SMART status. This program will be able to predict when your drive will crash.
But remember that your drive can still crash anytime from now onwards. So its best that you immediately take a backup / start shoping for a new drive (80 gb- costs 2300, 40 gb- 2000)
PS: If the drive is still under warranty, then naturally get it replaced- but its not that simple..
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