If you have a SCSI-controller on your Warp Engine, it should have DMA (direct memory access) which means disk access uses little or no CPU power (and with a decent not-too-old drive you'll get close to 10 MB/s speeds).
FastATA, and any IDE controller, use quite a bit of CPU during disc access.
So with SCSI (with DMA, there are non-DMA scsi controllers around too), you not only get faster disk access but also a generally faster system because of the extra CPU power freed by not using IDE.
