You can setup the drive in the Athlon. But my gut feeling is that the machine might have a 500MB limit. So the larger capacity might appear fine at first, but it will fall apart once you fill it beyond 500MB.

Easiest is to enter the drive parameters in the 486, boot up a Windows 98 disk and see what max. capacity FDISK will give you. It will likely be 500MB.

I keep getting HDD Controller Failure during POST. I have the CF Adapter on Primary Master and the jumper is set correctly. I've tried entering the HDD Parameters the AthlonXP gave me and also the same parameters that the actual IDE 407MB HDD has. Is there something I'm missing here?

Do you get the same failure if you attach a standard HDD?

Problem solved. Putting the SF Card into a USB HUB was killing it in DOS's eyes.