I protected an Excel workbook with Tools General
Options Password to Modify. If anyone goes into the
file folder where this Workbook is and tries to open it
they are prompted for the password and if they don't know
what the password is they can only open the document
as "read-only."

However, if the Workbook is accessed through the Website
(which it is linked to, to provide access without having
to drill down through the file folder structure) and
someone makes a change to the document, then uses the
Save As and drills down to the file where the document is
saved and saves over the previous version - it lets them!
It does not prompt for a password to save over the
previous version. Does anyone know why it isn't prompting
users for a password to modify the document? Anyone know
how to fix this? A simple workaround?