Use the NET USE command with an * instead of a drive letter. Then use the FOR command to go through the NET USE output to find the drive letter. You can then use that to copy the PST files to the remote location.
It might be easier to simply copy the files to a network share location such as:
<pre>copy “c:\users\%username%\my documents\*.pst” \\Server\Share\%username%\</pre>