I'm not sure whether this article has been mentioned here before,
but it definitely is worth a read:
http://www.usenix.org/events/fast03/tech/full_papers/anderson/anderson_html/
From the proceedings of the FAST '03 conference:
More than an interface - SCSI vs. ATADave Anderson, Jim Dykes, Erik RiedelSeagate Research
Abstract:
This paper sets out to clear up a misconception prominent in the storage
community today, that SCSI disc drives and IDE (ATA) disc drives are the
same technology internally, and differ only in their external interface
and in their suggested retail price. ... This paper attempts to clarify
the differences by illuminating some of these design choices and their
consequences on final device characteristics.
regards, tom lane