iMac like external monitor without disk

Hello tomillos,

Thank you for the question. Using your iMac as a display will depend on the model of your iMac and MacBook Air.

You can find the compatible models and the steps to enable Target Display Mode below:

Target Display mode Requirements:

4. What are the requirements for TDM?

A Mac notebook or desktop with a Mini DisplayPort or ThunderBolt connection

A ThunderBolt or Mini DisplayPort cable

A supported iMac with OS X 10.6.1 or later. The table below shows iMac computers that support TDM, the required cables, and the port of the computer to which you are connecting the iMac.

iMac Model	Cable Supported	Port on Source Computer
iMac (27-inch Late 2009)	Mini DisplayPort to Mini DisplayPort	Mini DisplayPort or Thunderbolt
iMac (27-inch Mid 2010)	Mini DisplayPort to Mini DisplayPort	Mini DisplayPort or Thunderbolt
iMac (21.5-inch, Mid 2011)	Thunderbolt to Thunderbolt	Thunderbolt
iMac (27-inch, Mid 2011)	Thunderbolt to Thunderbolt	Thunderbolt
iMac (21.5-inch, Late 2012)	Thunderbolt to Thunderbolt	Thunderbolt
iMac (27-inch, Late 2012)	Thunderbolt to Thunderbolt	Thunderbolt

5. How do I enable TDM?

Make sure both computers are turned on and awake.

Connect a male-to-male Mini DisplayPort or ThunderBolt cable to each computer.

Press Command-F2 on the keyboard of the iMac being used as a display to enable TDM.

Note: In Keyboard System Preferences, if the checkbox is enabled for "Use all F1, F2, etc. keys as standard functions keys," the key combination changes to Command-Fn-F2.

Find the full article here:

Target Display Mode: Frequently Asked Questions (FAQ)

http://support.apple.com/kb/HT3924#4

Thank you for using Apple Support Communities.

Best,

Sheila M.