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This is an ancient post with no relevance to modern systems.
Mixing SCSI and IDE drives can be a problem if you want the root disk to be scsi. The short answer is that you can if your BIOS lets you select which drive to boot (if not, you would have to boot from a floppy). Assuming everything else is in place, "defbootstr hd=Sdsk" should work. See SCSI for a full discussion of this.
Update March 2011:
If the question were asked at all today, it would almost surely involve SATA drives and the motherboard BIOS would absolutely allow you to boot (or try to boot) from anything you wanted..
I haven't seen much SCSI recently, but some still say SCSI has real advantages over SATA. My expectation is that in all but the most demanding RAID implementations, you'd be hard pressed to find any fault with SATA.
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