There are now many PC systems in common use that combine SCSI drives or devices with IDE/ATA ones. This is easier to do now that it has been in the
past, but still takes more work than using only one or the other by itself. For the most
part you end up having to do double the work since you have to configure two different
interfaces. Still, there are situations where it makes sense. For example, you may want to
add a SCSI optical disk or other device but continue using IDE/ATA hard disks, since they
are significantly less expensive than SCSI ones. Many people also add IDE drives to
existing SCSI systems to store infrequently-used large files, or for backup purposes.
In most cases, IDE and SCSI devices can be combined without too much difficulty. In
particular, adding non-hard-disk SCSI devices to an existing IDE/ATA system rarely poses
much of a problem. The difficulties seem to be largely confined to mixing SCSI and IDE/ATA
hard disks. In particular, many people who mix SCSI and IDE hard disks want to boot from
the SCSI drive, because it is probably the fastest one in the system. However, by default,
PCs will look for and boot the first IDE/ATA drive they see in the system, since the
system BIOS natively supports IDE/ATA and not SCSI. This causes fits for people who have
been using an all-SCSI system and add an IDE drive, and find the PC now wants to boot the
new drive.
There are a couple of ways to get around this. The easiest one is to use a system whose
BIOS supports booting from SCSI instead of IDE in a system that has both. This is
typically implemented via the boot sequence BIOS setting. Today, most newer PCs will support this feature, but some retail PCs
don't provide the boot sequence feature. Many older PCs don't have this setting either.
Another option is to set up your drives so that only the SCSI drive has a bootable primary partition. Configure the IDE/ATA drive
to only contain logical, non-bootable volumes in the extended partition. If you do this,
the system may boot from the SCSI drive because it is the only one that is bootable. Of
course, if the IDE/ATA drive already has a primary partition, you will have to use a
third-party repartitioning tool to change it. This also may not work on all systems or
with all operating systems; the boot sequence solution is preferable.
Finally, there can be complications if you try to use a SCSI host adapter and some
types of add-in IDE/ATA controllers in the same
PC. IDE cards like the Promise "Ultra" series appear to the system as if they
were a SCSI card. If there is also a real SCSI host adapter, which drives are recognized
first comes down to which card is seen first by the operating system at boot time. If you
have this configuration and the drives are being seen in the "wrong order", you
may be able to fix the problem by manually changing the various cards' resource settings.
Swapping the PCI slots used by the two cards may also correct the problem.
Next: IDE/ATA
vs. SCSI: Interface Comparison
