I have a iMac 2009 later with High Sierra OS, I am using my mini display port already to 2 other tvs. I need to connect a 3rd tv but to display a different display. Only other ports I have available is a Firewire 800 port and 4 USB ports, I understand that the display driver for the USB ports on this Mac and OS version there was a problem and will never work. What other option do I have to run another 3rd video output to show a different display.

  • There’s no possible usb-to-hdmi/vga/whatever dongle that will work. Display and audio require usb 3.0 speeds at least, and your 2009 iMac doesn’t have any usb 3.0 ports. Your mini display port is your only option. – Tom Hood May 19 '19 at 16:32

You dont say which 2009 iMac you have, but even the Late 2009 27" seems to support only one external display, up to 2560 x 1600. (From things like Everymac.com and MacTracker)

How have you managed to get two external displays running from 1 miniDisplayPort?

I doubt very much whether these machines have sufficient graphics hardware to drive 3 displays - 256Mb or 512 Mb Radeon 4000 series.

There may be third-party USB solutions, but they are likely to be annoyingly slow. It's USB 2, for starters, which does not have sufficient bandwidth/speed.

  • I have a minidisplay port to hdmi cable and connected to a hdmi splitter box. I have used all types of usb to hdmi adapters and none work. I was told there is a hdmi display splitter that sends out different display signals, im trying to find it online. – Rick May 19 '19 at 16:30

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