SCSI ID can be set from 0 to 15. ID 7 is used by the SCSI card, don't set you device ID to 7. You want to set your hard drive to ID 0 or 1 because they should boot up first. When your computer is booting up, the SCSI bios should identify what ID the hard drive is using or you can look at it in the device manger inside the Control Panel. Once you know that you can set you SCSI cdrom and burner accordingly. Remember you cannot set them to the same ID. Just like on the IDE bus, you cannot have two masters or two slaves on the same bus. The SCSI ID is set using hexadecimal code, check your manual to find out how to set it. SCSI 2 devices can be set from 0 to 7. SCSI 3 device can be set from 0 through 15.
The fastest computer is not necessary a good running computer.