Look on the back of the drive for a jumper. You'll see six prongs with the jumper connecting two of them. You'll have to look in the manual to find out exactly where that jumper needs to be in order to set it as the master on that IDE channel.
However, you're not going to want to set it as the master if your hard drive is on that same IDE channel. If you've got two IDE channels on your motherboard, set the ODD and HDD as masters on separate channels. Otherwise, you're stuck with the ODD as a slave unless you get a PCI IDE controller.
If your drives are on the same IDE channel, the reason why you're getting slow write rates is due to the limited amount of bandwidth allowed via PATA. The HDD is trying to send data through the channel to the RAM/CPU for processing while, at the same time, the RAM/CPU are trying to send the processed data through the same channel to the ODD for burning.