Orren: no, doesn't help, not even restarting my mac.
Visiting the file dialog:
Mouse click on target drive fails but...
If I select another application ( such as Safari or activity Monitor) then click back on the file dialog, it becomes active!
Are you sure your permissions for the drive are ok?
Here is a suggestion:in the finder, select the drive and then check the box that says "Ignore Ownership On This Volume". It will ask for your admin password when you do this (your maid system password is your admin password in most cases).