I've turned off all the auto-hibernation/auto-sleep settings in the control panel, and when the ext monitor is not plugged in the laptop doesn't hibernate. With the ext monitor it hibernates after about 20 minutes, even when the PC is in active use. The laptop's lid is closed.
I'm running W7 on an Asus UL30A, and the external monitor is an LG FLATRON E2041.
I'm running W7 on an Asus UL30A, and the external monitor is an LG FLATRON E2041.