Are you rebooting your Mac after disconnecting the monitor? If not, the OS probably thinks there are still two monitors connected. Probably have to go into Display Settings and tell it there is only one monitor.
Konica Minolta bizhubpress C1085 - Fiery System FS150 Pro
Fiery IC-308 v2.0 - CWS 5.8.0.39 SP2 on Windows 7 Pro SP1
Konica Minolta bizhub Pro C6501 (65C-KM V 3.0) - Fiery System 9
Xerox DC250 (DC250 V 1.2) - Fiery System 8e Release 2
Fiery Pro80 65-55C-KM PS Color Server 3.0a eu - CWS 5 (5.7.0.29) on Windows XP SP2
EFI CWS 6.1 (6.1.0.28) on Dell Studio XPS running Windows 7 Pro
AMD Athlon II X4 640 3.00GHz 4Gb RAM