Yes I have trouble with partition to partition copys as well, they either run like lightning (typical of all version of windows prior to win 7 sp1) or go into "slow mode", more often slow mode!.
Copys from same to same partition are still lightning unless I have stress on the system.
Copys from HD to usb 2.0, gigabit 2k router, and even the ramdisk (after the latest 13 updates from MS) are also experiencing this slowdown.
I am running windows 7 sp1 64 bit on a 3 tb seagate barracuda - system is a core i7 920 2.56 ghz with 16 gigs ram!
This seemes to have begun since either installation of the seagate goflex home 2 tb drive and/or upgrade to win 7 sp1 release, all file copies lost transfer speeds and test in control environment running nothing excessive and nothing in FOREGROUND demonstrates a differential spanning from .72 seconds to nearly 3 minutes to copy a 4 gig files from and too the various taargets mentioned.
I am aware of "bufffering" and thrashing the hard disk with multiple r/w's and the obvious does not seem to be apparent.
Duf