Why can't I sync a folder from another partition/drive on Windows?

Unfortunately, according the the Ubuntu One FAQ, you can't:

Can I sync folders outside my home folder?

No, you can only sync folders inside your home folder. This applies to both Windows and Ubuntu clients.

I'm assuming you have Windows on a C: partition and all other files on a D: (or other) partition? This is what I have set up and was looking for an answer, too. I'm not sure how effective One will be with this limitation.

Mounting a drive into a folder on your home directory will work. I mounted my fixed F: to an empty folder in my /Users/username/... directory using the directions listed here.

Use this link to get to Drive Management in Windows 7.

That worked just fine.