So what to do with this giant screen going to waste? That's when I found out about Target Display Mode. Certain vintages of iMac built since 2009 have the ability to function as a secondary display for other Macs that have Thunderbolt ports. All you need is a mini Displayport to mini Displayport cable. The only one I found online was this one from Belkin at Amazon for $22.
I connected the iMac to my Thunderbolt dock, which is already connected to my MacBook Pro Retina and, voilà, it instantly turned into a giant, beautiful second display. Now, you may say, what a waste of a perfectly good iMac, and I might agree except the iMac was going unused anyway and it's still completely usable as a Mac. All I need to do is unplug the cable and I'm right back in an independent iMac, all ready to go.
Plus, did you see that gorgeous 27 inches of screen real estate?
Now, I did have one tricky moment. The iMac's screen was way too bright compared to the MacBook Pro and while Apple's technical information said I could control the brightness from the MacBook's brightness controls, I could not, in fact. Nor could I control it using the brightness controls on a keyboard attached to the iMac. But what did work was a System Preference Pane called Shades, which is available as donationware. It allows you to set the brightness of multiple attached monitors individually.
Brilliant!
