Why didn't cable select work?

Question

I got roped into doing tech support for a friend of the family. Obviously I'd already failed to hide my powers, ala Penny Arcade.

Anyways, the guy bought a DVD burner OEM from Microcenter, and asked me to install it. So I stopped by before and thought I'd be slick and use Cable Select on the jumpers. I didn't get a chance to test it before it I had to leave, and it seems that this didn't work. I came back this week to investigate, and he explains he's confused how none of the software he downloaded was able to burn. So on a whim I switch it to explicit master / slave, and it starts working fine. Whoops. Well, at least it's not the extra crap he found and downloaded for free from the internet.

Why doesn't setting both jumpers to Cable Select solve this?

Answer 1

Was the IDE cable the old 40-conductor type? They were commonly made without support for cable select.

Pin 28 is grounded on the motherboard and connected to the Master drive. The slave has this connection removed, so it doesn't see a ground (it likely has a pull-up resistor on the drive, so it'll see a high level signal instead).