Changing startup screen resolution

After connecting and then disconnecting a second monitor, the resolution of the startup screen on my primary monitor is lower than it's native resolution. It is, I think, the native resolution of the secondary monitor. The native resolution returns after the startup is complete. I have deleted the "com.apple.loginwindow.plist", repaired permissions, emptied the trash, and rebooted. I have, in addition, used that procedure also deleting the "com.apple.loginwindow.plist.lockfile", but when deleting that file, I was unable to empty the trash because that file was "in use." Neither of these methods worked.

Does anyone have a means to reset the monitor to it's native resolution on startup?