I'm not really answering your question (sorry), but you can create a virtual hard drive from within VMware's interface. The file for the virtual hard drive can reside anywhere, including your external drive. It will start out small and grow up to the max size you specify when you create it. On the Windows side, it looks like a regular hard drive. In my case, my second drive mounts as the "E:" drive.
I find this useful, because the virtual drive does not take up my whole external drive. And the external drive can be formatted for Mac (HFS+) while Windows XP thinks the second drive is formatted for NTFS. In my case, the file for the virtual drive resides on an external drive that I uses for other Mac things.
Now, if you want to share data from both the Mac and Windows sides, instead of using a virtual hard drive (or a real hard drive), you can use WMware's Shared Folders feature to map a share point on the Mac side as a network drive on the Windows side. In my case, I have a folder in my Mac OS X user Documents folder mapped to drive Z: in Windows. Since it's not really going over a network, it is fast.
So if I use this drive in Windows, the files are immediately available in Mac OS X. If you access that folder in Mac OS X, the changes are immediately available in Windows. I can probably go the other way and connect from the Mac side to a share point on the Windows side, but I haven't figured that out yet.