When I starts my computer,sometimes it does not detect the hard disk.

When I unplug & plug the id with hard disk again it gets detected by processor easily.But this problem occours often and I go for the solution I mentioned above.

Tell me wats the problem is and hw can I fix it,, :

tell me wats the problem is and hw can i fix it
Were computers so definitive... impossible without the actual hardware to test. I can offer a general statistical breakdown of possibilities. For most of the cases of this behavior that I have encountered where the drive was installed correctly (jumpered/cabled), the culprit was a bad drive. Occasionally the controller has been at fault. A few cases involved compatibility with other devices on the same channel. Other problem candidates are cable length/type for interface mode being used, insufficient power delivered during drive spin-up, or BIOS interface settings mismatch with drive capability.