Okay, I found a solution, but it's a little more involved. I assume you are running Windows XP. If not, this may not work, or may need modification.
Here's what I did...
1. Created a folder on a drive called "Rock"
2. Created a folder on another drive called "Classical"
3. Set up both of them as network shares with the option "Allow network users to change my files" turned ON.
4. Set my Base Path in "Rename Files from Properties" to \\computername\
5. Set the directory rule in "Rename Files from Properties" to [Genre]\[Artist]\[Album]
6. Executing "Rename Files from Properties" will now move files according to their genre onto the appropriate drive
If you have more than 2 genres you need to control, you can make a new field in MC and populate it manually. In other words, create a field called "Location" (or something similar if this one is used) - for Classical music, put Classical in this field; and for Rock, Country, etc put the word Rock into it (or however you want to split them up). Then use [Location] instead of [Genre] on the directory rule.
The only problem with this method is that your music folders are shared on the local network and others can delete/modify files (assuming you are using Simple File Sharing). More security can be obtained by turning off Simple File Sharing and setting up permissions, but that is too complicated for this post.
I tested this and it worked on my system (XP Pro SP2).
Hope this help!