1. Make sure that jumper setting of your existing and new CD/HDD are not the same, otherwise both device cannot be detected by your system. Jumper settings is located at the back of the device.
Leftmost Jumper: Master
Center Jumper: Slave (left open in HDD)
2. Make sure that IDE cable is inserted properly, remember that pin 1 will always be positioned beside the power supply connection of the device. If your system wont boot or blacked out after you insert a new CD or HDD, try checking the cable position.