So, what about ATA disks. The thing with ATA disks, is that they are what they call "blocking" devices. This means, that only one disk per channel can communicate with the controller at the one time. Since most computers have two channels, two devices can talk to the controller. (SCSI is also referred to as "Non-Blocking device"). This is the reason why programs like Nero burning rom will recommend you to put your primary CD on one channel and your writer on a different channel.
An average ATA harddrive has a very good latency, but a rather high seek. This means that sequential transfers can be faster than on SCSI disks. But when you run a database on such a thing, it turns bad.
Secondly, ATA harddrives have a very low MTBF (Mean time between/before Failure), which means that the harddrive will not work for very long without critical errors. This is the reason why servers containing ATA harddrives usualy have only 1 year of waranty... and if it says 3 years, usualy this waranty does not include the Harddrive. But as long as you are running sequential transfers... ATA can a good solution. Don't burden your HD with a database though... certainly not on the harddrives used in portables, which have an even higher seek.