Problem of Connection of two HDD

sw123456

My computer connection is:
Primary IDE- Master 160GB HDD (jumper setting: cable select) + Slave 80GB HDD (jumper setting: slave)
Secondary IDE - DVD Rom
I used this connection for three months.  But one day, Windows XP of the 160GB HDD cannot started.

I changed the setting to Primary IDE- Master 160GB HDD (jumper setting: cable select) + Slave DVD Rom (jumper setting: slave)
Secondary IDE - Master 80GB HDD (jumper setting: master)
Still not work.
Then I disconnect the 80GB HDD, the Windows XP of the 160GB HDD can start now.

I am confused why the connection of 80GB HDD will prevent the 160GB HDD to boot up now, but not on day-1 when I set up the 160GB HDD from the old 80GB HDD, and change the 160GB HDD as the boot disk.

Can anyone teach me how to connect the two HDD, as I wish to use the 80GB HDD as storage.  Thanks..........