Disconnect backup HD. Computer freezes

I do not realy know if this is a software ore hardware problem.
Me computer is with AMD64 processor. 2GB Ram, 2 Hard disks, Win. XP2SE.
The HD with Windows installed is a SATA with drives C-D-E and plenty of space. Installed to SATA0 on 0 (Primary ATA Channel)
All programs and data is installed/stored on this HD.
The second HD ATA, with drives F-G was connected some time after i got the computer and is installed to IDE2 on 0 (Secondary ATA channel) and is only used for backups.

Now, when I disconnect the second HD, the computer boots fine, but as sun as I click on an icon, the computer friezes.

Also, when i run the freeware win. analyseprog. SIW and klick Storage Devices, the computer freeze.
Running chkdsk /F/R does not help.

I have tried to change IDE Channel, but it does not solve the probelm.
I would like a good explanation on this problem.
Regards from Anton