File copy takes 100% of processor time

I’m running NT4.0 with SP6a. I recently changed from 40Gb hard drives to 120Gb hard drives. Since then, any file copying or moving slows the server down to almost zero. Using the system monitor, I see that usual processor time utilization is around 2% to 6%. Once I start copying files from one hard drive to another (both on server) the CPU time utilization shoots to 100%. Which makes it unavailable for other processes.

Any ideas?