Hardware

I don't have enough information to determine what is the root cause of the problem however you may want to try robocopy. Robocopy is built into Win 7, just go to a cmd prompt, robocopy /? will display all the options and there are alot.

The most common ones I use are:
robocopy source dest /e /r:2 /w:5 /np /log:path\filename /mt /xo

source and dest are the paths or urls to the folders.
/e copy all folders and subfolders, even empty ones.
/r:2 Retry 2 times on copy failure (can be set to a different number)
/w:5 Wait 5 seconds between retries (can be set to a different number)
/np Don't display file completion percentage
/log:path/filename Keep a log of actions
/mt Use multi-threaded copy
/xo eXclude Older (Overwrite old files with newer files)

Example: robcopy c:\data d:\backup /e /r:2 /w:5 /np /log:c:\robolog.txt /mt

Review the logs for any errors.