Reducing heat by adjusting the lid
Having the lid closed or open does not impact the airflow of your MacBook Pro (MBP). Ambient air is never in short supply and the "exhaust" (where the hot air picked up by the blade fan(s) are expelled) is located at the back of the unit (the black stripe on the newer models and the grey stripe on the first generation machines).
Some claim that keeping the lid open allows the chassis to cool down, but these claims have never had any real data behind them (simply anecdotal evidence and unsubstantiated rumour). Personally, I have a 2006 MBP that was eventually relegated to being a desktop. I ran the system for well over a year with the clamshell open, and then closed, and saw no different in CPU/CPU temps (nor any variability in the other heat sensors) as a result. Moreover, the newer models have far more efficient blade fans and would suffer even less from the effects of heat (additionally, the new Intel chipsets run cooler than their predecessors). You can conduct your own testing of course. I had replaced the heat sink, re-applied a sane amount of Arctic Silver thermal paste, and used Lobotomo Fan Control daemon to monitor my system's temperature.
Disabling the internal display, properly
There are two ways to connect a secondary display to your notebook without enabling the default screen (note that this does not mean dimming it, but rather turning it off).
- The first is to connect the display with the lid closed when the machine is powered down. Once the external display is connected, turn the machine on—it will detect the secondary display while leaving the internal one turned off. This will alleviate the issues you have with "dragging things off the screen" as the system will constrain your desktop to the one, active monitor.
- The second is to connect the secondary display when your system is awake and active, and the clamshell open. Make sure the notebook is plugged in. Once you connect the secondary monitor, the system will recognize it. Once it does, close the lid on your notebook. The device will be put to sleep. Once that happens, move your mouse, or hit a key on your keyboard to wake it. Once it wakens, it will enable the secondary display, but not the internal one.
Additionally, you may set the attribute to leave the system in the sleep state even if the clamshell is opened. You can do this using pmset, a local utility found on OS X that actually handles all your power settings. You can accomplish this with the following command:
sudo pmset -a lidwake 0
The -a, -b, -c, -u flags determine whether the settings apply to battery ( -b ), charger (wall power) ( -c ), UPS ( -u ) or all ( -a ).
To reverse the command, set the 0 to a 1. These settings are persistent, in that, they do not need to be re-applied every time the machine's power is cycled and are saved to the file: /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist
Run sudo nvram boot-args=iog=0x0 and restart. It restores the behavior on 10.6 and earlier where the internal display is not turned back on if the lid is opened in clamshell mode.
You can delete the variable with sudo nvram -d boot-args.
Other related questions here and at Super User.
Best Answer
With some care it can be done safely. It depends by and large on what kind of loads you're placing on the MacBook when it's being used like this.
The MacBook Pro's ventilation operates optimally with the lid open. Closing the lid restricts airflow out of the vents that run along the inside of the hinge on the laptop as seen here:
When the lid is closed, there's a slight gap along the bottom that's used as an exit route for the hot air coming out of those vents as seen here:
If you're closing the lid, you should ensure that the entirety of this exit path along the bottom of the laptop is free from obstructions. That means not placing the bottom of laptop on a flat surface. Generally a vertical stand like this is employed if you want to operate the laptop with the lid closed and ensure ventilation remains adequate:
You should monitor the temperature sensors of your laptop to ensure your workloads don't push the unit beyond normal operating temperatures when you're using it with the lid close. You can use something like iStat Menus to keep an eye on critical, system operating parameters such as CPU use and the various temperature sensors in your MacBook to make sure they stay within their safe operating limits.