IE11 - Unable to add website to Favorites

I found the answer in this article.

Somehow my registry had entries that were mapping the Favourites folder to a disk that did not exist.

By correcting the "Favorites" keys in these locations and restarting IE the problem was resolved:

HKEY_CURRENT_USER\Software\Microsoft\Windows\Current Version\Explorer\Shell folders
HKEY_CURRENT_USER\Software\Microsoft\Windows\Current Version\Explorer\User Shell folders
HKEY_USERS\.default\Software\Microsoft\Windows\Current Version\Explorer\Shell folders
HKEY_USERS\.default\Software\Microsoft\Windows\Current Version\Explorer\User Shell folders