Using one partition for Win one for Mac on USB

I want to use one partition of a USB terabyte drive for backing up the Mac and the other partition for backing up the Win XP virtual machine. But under Devices/USB, I seem to have to assign the whole drive to one machine or the other. Thus, automatic scheduled backups for both won't work. (I have formatted one partition for Mac OS (journaled) and the other for NTFS.) I'm using SuperDuper! for backing up the Mac machine.

When I have the whole drive assigned to the Mac, I see In Windows Explorer (not IE) the Win partition as a virtual drive W: and I can see its content, but I can't write to it, not even in DOS ("access denied").

Is there any solution? The ideal would be a way that Parallels could use Devices/USB to assign just the Win NTFS parition to Windows. Alternatively, could there be an automatic script on the Mac to switch the Devices/USB assignment for both partitions to the right machine before the appropriate automated backup runs?

It would be helpful if you could notify me at [email protected] if you have a reply.

Thanks,
Harry