From what I understand from some disk manufacturers, the servo control logic on these disks are less sophisticated than on SCSI drives. So, when the drives are packaged together, as would be seen in a RAID enclosure, the heads can tend to "wander off track" more easily than with SCSI drives causing corruption or loss of data.
Also, the same manufacturer has somewhat stated that it won't be until the next generation that they will bring the logic up on these disks to match that of SCSI.
There are a number of engineering differences between ATA and SCSI drives. In general, new innovations are put in SCSI drives first and later make their way to ATA drives.
The phenomenon you describe is called rotational vibration. It is characterized by the interaction of adjacent disk drives within a subsystem, resulting in higher error rates. That said, I'm not convinced how much of a problem this really is for ATA drives. I suspect the effects of rotational vibration may not be that large.
A bigger issue is the amount of profit in SCSI drives. SCSI drives are much more profitable than ATA and there is little doubt that disk vendors would prefer to get more margins for their products.
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