My external drive is housed in an old SyQuest removable drive case. Internal drives are always zero, so if you're fitting an internal drive you need to make sure it is set to zero. And if you're hooking up an external, you need to make sure its number is not zero. Well, externals should have a 'SCSI selector' button at the back of the case so you can easily see what number it has, and change it if necessary. If you can't see how to set the ID, think twice about buying the drive.
Bare drives are trickier.
Try asking the supplier of the drive, but if they don't know, you'll have to do some detective work. Bare drives often have an ID of zero, so if that's want you want you could just try it and see. If it doesn't work, you'll have to dig deeper. This isn't as complicated as it might sound when you get the hang of it. The vertical lines represent a jumper clip connecting one pin with its partner.