To use the new 27-inch iMac as an external monitor requires a very specific cable to work: a Mini DisplayPort to Mini DisplayPort cable, to be exact.
It does not work with a HDMI cable or a DVI cable, even when they are strung between a pair of Apple-branded Mini DisplayPort adapters.
There seems to be a lot of confusion about the issue on the web and even at Apple’s retail stores.
To use the new iMac as a monitor, it must enter “target display mode,” which appears to happen automatically when the correct DisplayPort cable is plugged in.
There appears to be only one cable on the market capable of connecting compatible devices to the new iMac: Belkin’s Mini DisplayPort to Mini DisplayPort cable, which is available from Apple’s retail stores for $29.99.
The store didn’t have any in stock, so the staff happily sold me a pair of Mini DisplayPort to DVI Adapters ($29 a pop) — and told me to string them together with a standard DVI cable.
I had to go to another store to find a DVI cable to connect them (another $20). At this store, I found a pair of DisplayPort to HDMI dongles, so I picked them up (another $15 each) and a $12 HDMI cable to connect them.
Neither of these MacGyver solutions worked.
I’ll try to pick up one of the Belkin cables tomorrow and will post the results here.