Just noticed this as I'm diagnosing the problems....

I first thought "there's my trouble! Crimp/puncture in the cable!

...but BOTH cables have a notch out on the 80 wire cable around wire 65. It is one wire width, very precise, about 1/8th of an inch long.

Anyone know why they'd bother to literally disable that particular wire?

They're supposed to?

Probably so they could make sure that whatever that wire was for, it didn't. Many IDE headers are missing one pin. Many IDE cables have one hole blocked.

This is a way to not block the hole (making the cable not work on headers that don't have the pin removed) and still disable the connection.

I don't remember what it did/does.

interesting that the pin is blocked as well.

Interesting you mention that. When I was having problems with the kids' computer starting up months ago, I happened to notice a similar hole in the ATA cable and I thought that might be the problem. I changed out the cable and it didn't help, so I put the original back, reconnected everything again and it was okay.

Yes, I noticed that too on my Quicksilver 2002's stock drive cable. I'm assuming that it's intentional...

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Freya says, 'Hello from NOLA, baby!' (Laissez bon temps rouler!)

40-wire cable-select cables have such a hole. It forces cable select mode. It reverses the usual attachment order of Master and Slave drive on the cable, putting the Master drive in the middle. If you use that cable in the wrong configuration, you can get odd results like having two drives inadvertantly set for Master.

Those cables are (thankfully) no longer made. Modern 80-wire cables work with both cable select and Master/Slave jumpered drives and are much more reliable as well as backwards compatible.

Standard cables will have pin #20 blocked for the same reason that there's usually a tab at the top of the connector -- it's to make sure that the orientation of the cable is correct. Some controllers were made with the pin in place and for those, you'll need a non-standard cable with the hole not blocked (they're not hard to find).