In my experience the IDE can be troublesome on some A4000Ts. A fool proof solution is to use ROMS from the A4000D computer if you want to disable the on board SCSI. This works quite well. I bought a machine that had been configured in this way and I could not figure out why at first. It makes the IDE much for reliable at least one of my A4KTs. Also you may not be waiting long enough for the IDE. Connect the IDE drive and wait several minutes looking at the black screen, the machine may still boot if you have it configured properly!
Someone else will probably tell you the IDE on there machine works just fine, but one of mine sure doesn't seem to:-D
What I ended up doing was to connect an IDE CD-R drive as master on the IDE buss (without a hard drive) and use the SCSI for my boot and system drives. This way the machine seems to work fine with the correct ROMS installed. Also make sure you terminate everything properly if you are using SCSI.
YMMV Good Luck,
Jeff
