Having started this thread, I wanted to report back on connecting my new iMac to an older NEC MultiSync P221W display. I used a Thunderbolt-to-DVI cable which seemed to work fine -- well, sort of. After restrating, the iMac recognized the second display. I then configured System Preferences to use the second display as an extended screen (not mirroring).
However, two issues have surfaced: (1) Whenever I attempt to drag an image in LR from the iMac to the second display, the image won't "snap" into position on the second display. I literally have to rapidly drag the image across the screen in order for it to move it to the second display. This seems to work only after several attempts. (2) When the image is finally displayed on the second display, it will not automatically fill the screen.
Both of these issues never occurred when the second display was connected to my old MacBook Pro. Hopefully someone can offer a suggested remedy. Thank you so much.
Kit