Windows 7 Ultimate file copy problems

I've been having issues since installing Windows 7 Ultimate 64-bit copying large files to and from my hard drive, an external USB hard drive,  and a network NAS.  I made a few DVD ISO for archiving on my PC using ImgBurn and then trying to copy to my DNS-323 NAS or an external hard drive.  When I copy large files (3+ GB), the source and destination checksum don't match.  So I delete the new copy on the NAS and try again and then the destination has yet a different checksum still different from the source.

I don't know if it my checksum program having issues or Windows 7.  I have used both HashTab v3.0.0 and sha1deep (both free) without success.  I also had trouble trying to copy a 6GB download from Direct2Drive.  The only way I am able to consistently successfully copy the large files is to use a command windows and use xcopy.

At first I thought it might be bad RAM but I ran memtest86 for approx. 18 hours or so and there were no reported errors.

The NAS and the PC are connected via network cables (i.e. no wireless) through a 5 port DLink gigabit switch.  My PC is a Core 2 Duo E8400 with 8GB of RAM and a 500 GB HDD.  The NAS has dual 1.5 TB drives in a RAID 1 array and the external USB HDD is also a 1.5 TB drive.

Has anyone else experience file copy problems with Windows 7?