I can't tell you if it was the Monoprice display or the wacky way you have to connect both a DisplayPort and USB connection to drive full 2560x1440 resolution on it and the other non-Thunderbolt monitors out there. I tried both the Monoprice adapter and the Apple Mini DisplayPort to Dual-Link DVI Adapter and had similar problems.
What were the problems, you ask? Well, sometimes the monitor would just blink off. It would shut off, completely hiding whatever I had in the window, and the only way I could get it back was a restart -- or three. On restart, all the window settings would go wonky and I'd have to go into display settings to re-setup the displays after each reboot, which would cause more wonkiness, and need another reboot. Plus, Parallels hated it with a passion.
It. Was. A. Nightmare.
I called Monoprice and they helped me through a variety of tests, but the monitor would just keep shutting down or losing its settings. Finally, I yanked the damned thing out (well after the return period), and bought an Apple Thunderbolt display (that kicked my wallet in the pants!), which connects via Thunderbolt rather than DisplayPort (yes, the very same plug on the back of the Mac), and works perfectly. Absolutely. No. Problems.
As it turns out, the Monoprice display doesn't appear to have been bad -- at least at 1920x1080 resolution. My wife saw it sitting there very much unloved and immediately adopted it to replace her old 20-inch monitor. She likes the bigger screen, doesn't need the extra resolution, and the monitor has been working fine ever since.
The moral of that story is don't try getting extra resolution out of a cheaper 27-inch monitor on a 4-display iMac using a two-headed weirdo adapter. As tough as it may be to swallow, pony up the extra cash and get the Thunderbolt display.
So, let's start this over again, enumerating the displays and their interfaces.
At the far left is a Vizio 24-inch TV connected via HDMI to a DisplayLink adapter, which plugs into an iMac USB port. Dead center is the iMac itself with its 27-inch display. To its immediate right is the 27-inch Apple Thunderbolt display, connected via Thunderbolt. And at the far right, turned 90-degrees on its side, is one of my original 24-inch monitors with DVI on the monitor side, going through a DVI-to-DisplayPort adapter into the iMac's second Thunderbolt port.
Here's another view of my setup from the front, with each monitor labeled so you can see what's what:
Oh, and remember the irony I mentioned at the very beginning of this article about the iMac not having enough Thunderbolt ports to drive another DVI monitor? As it turns out, the Thunderbolt display is also a Thunderbolt hub and brings additional Thunderbolt, USB, and even an Ethernet port to the party.
I'm a big fan of hanging my monitors off arms so I can easily adjust them to whatever task I'm doing. So if you're curious about how all this looks from the back, here you go. This is the back of the Vizio TV, hanging off a very cheap monitor arm I picked up from somewhere (I don't recall, or I'd tell you). I had two, the other is holding the other 24-inch display.
You can see the iMac from the back. You can't really see the Apple Thunderbolt display, but you can also see the 90-degree rotated AOC 24-inch monitor peeking through.
Next is a view of the iMac and the Thunderbolt display, on very robust (and relatively inflexible) Ergotron arms:
That Thunderbolt display is a beast. It's actually heavier than the iMac and thicker at the edges. Also, converting it from a desktop model to a VESA arm model was another $50 in Apple parts and about two hours. It was not a small job.
Finally, we walk our way all the way around the back of the desk and you can see the vertical monitor. That monitor is vertical because there's just enough room (when vertical) for me to get by and to my desk. If it were horizontal, I wouldn't be able to easily get to work. Also, I've found the vertical display to be very helpful for certain kinds of output (TweetDeck is a great example). In any case, here's the back of it all:
Stay tuned. I expect to tell you more about how I got Parallels to work in this configuration, and more (including how I took the four-display picture from the front and managed to light the thing).