I have followed the instructions posted here:
but my new 27" iMac 5K will not recognize my 2nd monitor. I am using an Apple DVI to Thunderbolt adapter. The monitor is a ViewSonic VP2365wb (it's several years old but works fine). I have restarted, re-plugged the connection, detected the display. Are some DVI connections different from others?
iMac, OS X El Capitan (10.11.3), 27" Retina 5K, late 2015, 24GB Ram
Sorry, wilddezign, but I'm having the identical problem. If you find a fix, let me know. My monitor only failed within the past few days. 1st with a Thunderbolt 27" display connected (via Thunderbolt) to my iMac 27" 5K. Switched to test the iMac with an old 27" Cinema display (which does work when connected to my Windows machine), and it failed with the iMac. In either case, monitor does not light up and is not recognized (though the USB port on the Thunderbolt monitor is recognized). Tested the Thunderbolt display connected to my Windows machine and it worked. So the problem is NOT the 2nd monitor. It is in OS X or the iMac. (08-27-16)
Issue resolved. When I disconnected one of 3 external hard drives (a RAID device), the 2nd monitor displayed perfectly. I was unable to reformat that drive, so I have dumped it. No problem at all when the other 2 external HDs were reconnected.
2nd monitor not recognized by iMac 5k
