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For troubleshooting on this, take a look at this article:
Intel-based Macs: Resetting the System Management Controller (SMC)
A computer that supports target display mode does not switch into or out of target display mode as expected.
For more information on this, take a look at this article:
Target Display Mode: Frequently Asked Questions (FAQ)
Make sure both Mac computers are turned on and awake.
What are the requirements for TDM?
- A Mac notenook 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.
Best of luck,