[url=http://www.pcguide.com/ref/hdd/if/ide/confCS-c.html]LINK[/url] said:
Unfortunately, regular 40-conductor IDE/ATA cables don't support cable select. (Why this came about I do not know, but I suspect that some bean counter determined they could save five cents on each PC by doing this.) So to use cable select you need a special cable, and these are of course non-standard, making them a special purchase. Also, many people don't understand cable select, nor do they realize it needs a special cable. If you set both drives to "CS" and then use them on a regular (non-cable-select) IDE cable, both drives will configure themselves as "master", causing a configuration conflict.