Daniel -
If you check Ctrl-Alt-Del and look under Processes, and sort by CPU, then start the file copy - what process is at the top of the list, using most of the processor cycles?
I can't think of anything RAID-related that would cause those kinds of speed, without also slowing everything down to a crawl - it would take half an hour just to boot Windows, at those speeds.
Can you copy a file to the same folder without any slowdown?