If you have an ide disk on your computer, then you can't enable the scsi bios. If you boot with the scsi disk, you can't use IDE with it. If you have an ide disk, do not boot from the scsi but from the ide.
With Windows 98 and later I have never had any trouble using both IDE and SCSI on a single machine, I boot with SCSI and have always Used Adaptec cards (2940UW, 2940UWPro, 19160). You may run into conflicts between the controllers, but they are almost always resolvable.
Also, are you sure that the drive itself is functioning?
A bad HD can still ID to the system, and just not work.