Most SCSI drives change IDs with jumpers. The hard drive with either have a graphic on the drive listing the jumper configurations, or you will need to go to the manufacturer's website to look at the jumper options.
Depending on the type of drive you have (IDE for example) your issue may be a master/slave setting. Also set by jumpers and info will be on the drive or at the website.
If you can tell us what kind of drive you have (manufacturer and drive type) plus which machine you are using, myself or someone else should be able to get you a more solid answer.