I'm not sure that Media Center can handle whole drives directly, eg. when you try to access them by drive letter, and directly from their root directory. Rather it needs the shares to be in subfolders. That applies as well when adding the shares manually, it's clear that it asks for directory names. Maybe you can try to rearrange your SMB shares according to this somehow, and rather than sharing the whole drive, specify only the subfolders (or a main subfolder) to be shared.