You wont need MacDrive.
If you create your VM in Fusion and place it on your external drive it will all be stored there. Keep in mind that VM is exactly that, Virtual. So what happens is Fusion creates a file (called VMDK) which is a virtual hard disk. You specify the disk size in Fusion (which cannot be greater than the size of your physical hard drive).
Once you get XP loaded on Fusion, you can put the network adaptor in "Bridged" mode which will make the XP VM act like another machine on the same home network you have.
As for sharing files, Fusion has a "Sharing" option that allows you to share OSX folders in your VM. When you share those folders, they appear as a network share (
.host\shareName) thus allowing you access to them. You could also do the normal file sharing since you have your VM network adaptor as bridged, but the option is up to you.
Hope this helps you understand the virtual vs physical and how you have to think in 2 worlds to understand it.