If you have two different scsi controllers in your machine (in my case, I
have a builtin Adaptec 7896 chipset, and I have an old NCR53C810 controller
as well), the Redhat installer will arbitarily pick one of the scsi
controllers as the first controller.  In my case, it decided that the NCR
scsi controller was the first controller, while the BIOS and the Linux
kernel (if both controllers are compiled into the kernel) both pick the
Adaptec as the first controller.  If you have disks on both controller, it
means Redhat might put the /boot or / on a partition that the BIOS won't
boot from.  If you can't figure out which is the first controller that the
BIOS and kernel will see, you should ask the user which scsi controller to
use as the first one.