SCSi and atapi relationship

When you look at the SCSI icon in the control panel, there is a line or two about the atapi bus and the cd rom. What is this about? I’m getting event id 9 errors on a system that uses SCSI HDs and atapi cd rom. If I open the cd and leave the tray open, the system works fine except for the id 9 errors posting. If I close the drawer, the system locks up from time to time and even if it is not locked up, I can hear the cd making noises like it is looking for a disk. I have already replaced the cddrive to no avail. Anybody seen this?