OK, assuming that this new drive is for additional storage and your 80GB is still the primary with your OS…
Make sure that the drives are “jumpered” correctly.
If you are using the CS setting, make sure that both drives are jumpered to the CS location and that the 80 is on the end of the cable.
If you want to use Master/Slave, jumper the 80 to master and the 500 to slave. I would still keep the 80 on the end of the cable.