I'm able to see the external USB drive on the Mac mini here in the house from both Windows and Linux by creating a UNIX-style symbolic link to the (note the Mac GUI based Alias you can create looks similar, but I don't believe it will work). It does mean a one time trip into the Terminal and the command line, but it's not terribly painful
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Fire up Terminal on the Mac. Since the Mac wants to share the home directory, that's the simplest place to put this symbolic link, which will look like a folder in your home directory, though it's actually the entire external drive.
To create the link, use the "ln -s" command. The syntax is:
ln -s /Volumes/NAME-OF-EXTERNAL-DRIVE LINK-FOLDER-NAME
Replacing NAME-OF-EXTERNAL-DRIVE with the name assigned to the external drive and replacing LINK-FOLDER-NAME with the name you want the folder to have when you look in the home directory.
A folder with that name should now appear in your home directory. When you open that folder on the Windows box, you should see the contents of the external hard drive and be able to read and write from the drive.
At least, that's what I've just gotten running here and Windows and Ubuntu Linux seem happy about it.