The drive mechanisms are EXACTLY the same between ATA and SCSI drives except for the high-speed 10,000 and 15,000RPM drives, which run very hot. There is virtually no reliability advantage to SCSI Performance can be better with SCSI due to tagged command queuing (TCQ). However, Windows is not exactly optimized for great I/O performance, which means that TCQ might not buy you anything. Also TCQ really comes through under heavy workloads and is best for high-end transaction processing. An array of 7200RPM ATA drives should perform very well. Use RAID1 or 0+1 for best performance.
I'd say it's safe to save a little money and go to ATA drives for user files. Do you need hot swapping? If so, make sure you can get it in the subsystem you buy -- that's not a function of the drive but the RAID controller and the bus electronics.
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This was first published in September 2002