I think it's the Atlas drive that is not very CSPPC friendly. I've had similar problems in the past with that same drive, and once I switched to a Seagate Barracuda problems dissapeared. I've had best success with IBM U160 or better SCSI drives and CSPPC. They always work fine with CSPPC and would give speeds close to the theoretical limit (30 to 35MB/s, so pretty close to the 40MB theoretical limit).
EDIT: Fujitsu U160 or better SCSI drives also work great with CSPPC, from my experience.
