Macbook doesn't display on external screen until after I log in (Yosemite). How to fix? I have a macbook pro (mid-2011) and an HP Pavilion 22bw (prob inconsequential) connected via a Thunderbolt DVI Adapter.

Ever since installing Yosemite, I now need to open the macbook pro and log in via its own screen, rather than the external monitor.  It simply does not display / send any signal to the external screen until AFTER I'm logged in.

Does anyone else have this issue?  Is there any other option?  This is highly inconvenient with how my workspace is set up.

Thanks!
To Fix this issue do the following while your external monitor is plugged into the MacBook:

Open Displays in System Preferences
Click on Arrangement at the top of the window
If Mirror Displays is checked, uncheck it
The position of the two displays should now be shown in the Displays Window
One of them will have a white bar at the top.  This is probably your MacBook
Click on each display in the window and you will see the relevant display framed in red.
If the white bar is at the top of your MacBook display, move it to the top of the external monitor display.
Then click Mirror Displays again.
This should resolve the issue