When Apple introduced its late-2009 iMac systems, it was clear the large built-in display on these systems could not only be used for the accompanying computer, but similar to the functions of Target Disk mode for hard drives, the display could be used for another Mac.
While Target Disk Mode requires you to restart the system into a special mode, the use of Target Display simply switches the monitor to accept input from an external source, rather than the running system, just as if you had unplugged it for alternative uses.
To do this, first check the model of your iMac to ensure it was made in late-2009 or later, and then simply press Command-F2 after connecting the systems with a compatible cable. Note that on most Macs, the F-keys are assigned to a system function by default, so you will either have to revert this in the Keyboard system preferences, or hold the “Fn” key in addition to the Command key before tapping F2.
When you press this hotkey, the system will remain running, but the display will now be accessible through the Thunderbolt connection of the iMac, smaller systems such as laptops to make use of the larger display with the use of a Thunderbolt cable.
Troubleshooting
iMac not switching to Target Display mode
If your iMac supports Target Display mode but does not enter it, then try resetting your iMac’s SMC. Additionally, for older iMac systems, be sure you are using OS X 10.6.1 or later. In addition, be sure your have first connected your iMac and secondary computer before entering Target Display mode.
Finally, be sure you have applied all software and firmware updates for your iMac that Apple has made available. To do this, go to the Apple menu and choose Software Update, and then install any updates available, especially those for SMC firmware or OS software.
Ensure compatible connections
Even though Thunderbolt connections contain DisplayPort data, Target Display mode on a Thunderbolt-equipped iMac not work with DisplayPort connections. Therefore, if you have a DisplayPort-equipped Mac and wish to use the mid-2011 (or later) iMac’s display as an external monitor, you will not be able to.
For iMacs before the mid-2009 model, you will not be able to use a Thunderbolt cable for the connection, so ensure you have a proper mini-displayport cable. Note that the Thunderbolt and Mini-Displayport connections look the same, so differentiate these by the lightning-bolt symbol for Thunderbolt, vs the square bracketed symbol for Mini-Displayport.
Directly connect the iMac
While Thunderbolt supports daisy-chaining of devices, be sure to connect the iMac directly to the secondary Mac.
Media and brightness controls
When in Target display mode, you should be able to adjust the iMac’s volume and display brightness; however, keep in mind these will be adjusted from the iMac’s keyboard, and not from the one connected to the iMac.
Sound not playing through the iMac
If you have Target Display mode set up but no audio is playing, then go to the Sound system preferences on your secondary Mac and ensure the iMac is selected as the sound output device.