Board Thread:Online Discussion/@comment-37.60.131.230-20150322084258/@comment-6881629-20150322104100

If your Steam folder is inside your Windows folder it could be the user control protection preventing the second disk from making changes (but i would think you'd get a notification about it), if not and no one else answers who's had and fixed the issue you might get more help from the Official ESO Forum and their technical support section.