You would need a modded version of IntelPIIXATA.kext, check the Legacy one that comes with Chameleon 2 or the one (with a binary) made by Dune at insanelymac.
Check the CPU usage while the copy takes part, it sounds like a DMA problem, but I got no clue why it happens, are you copying to the same disk or from CD to disk or other? it should work without troubles in AHCI mode, I got a P35 (Gigabyte) board before and it was working in AHCI without any troubles