Seagate 500Gb hard disk problem (causes slow bootup and is not detected)

I have a system with the following cofniguration:
Intel Pentium 4 3.0Ghz HT
Intel D945GCCR Motherboard
2 GB DDR2 SD RAM 667 Mhz
Windows XP Pro
3 hard disks - Seagate 80GB SATA, Western Digital 320Gb SATA and a Seagate 500Gb SATA. All are 7200rpm ones. The problem is with my 500gb one.


One morning when I booted up my sys, it din't boot properly.There were no problems till then. First a steady cursor came at the top left of the monitor. Then after some 30-45 seconds, the BIOS screen came up (which usually comes within a few secs). Then the Microsoft Windows XP loading screen comes up and it takes nearly 5 min at this stage. Then only did my computer get into windows. At this point both my 320 as well as 500 HDD's were not detected. When i restarted and checked in the BIOS also, they were not shown. I unplugged the SATA cables from those two hard disks, then my computer boots up properly without any delay or the cursor showing up.
Now I tried connecting only the 320 HDD alongwith the 80gb (which contains the OS) and it worked fine. When i try connecting the 500 HDD, again the same problem comes up. Slow booting and then when I get into windows both the 320 and 500 hdd's don't show up.

Please help me with this situation. I have a lot of data on the 500gb hdd and really need to access it badly. I got no clue as to how to handle this situation.

Thanks