A second life as a second screen
Can I use my old iMac as a second screen alongside my new one to extend my desktop across both machines? Graeme Minto Target Display Mode is the easiest way to do this. You connect the two iMacs with a Mini DisplayPort or Thunderbolt cable and then press ç+ @ on the old iMac. The new iMac will see it as an external monitor after that. Macs older than 2009 don’t support Target Display Mode at all, Late 2009 and Mid 2010 models must have a 27-inch display. With Mid 2011 and newer models, Target Display Mode is available on both screen sizes (but not at all on Retina 5K models), and the iMac whose desktop you’re extending will need a Thunderbolt port. If either Mac doesn’t meet these criteria, Target Display Mode won’t work.
You could still do a very limited sort of screen extending using Air Display (£14.99 avatron.com), but it sends screen updates over the network, so you get much lower frame rates than with a dedicated video cable. This is good enough for desktop apps, if you are fairly tolerant, but it’s too jerky for full-screen video or games.