Is the StarTech.com USB32HDPRO adapter compatible with older versions of Windows?

Yes, the adapter is compatible with Windows® 8/8.1, 7, Vista, XP, and even Windows 2000.

What maximum resolution can I expect from the StarTech.com USB32HDPRO adapter?

The StarTech.com USB32HDPRO adapter supports a maximum digital resolution of 1920x1200 / 1080p.

I have no idea. I don't use Mac. But I can say it has done exactly what I needed it to do and done so flawlessly.

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StarTech.com USB32HDPRO USB 3.0 to HDMI External Multi Monitor Video Graphics Adapter for Mac & PC - DisplayLink Certified USB Video Card - HD 1080p

Connect an additional HDMI display to your Mac or PC with USB 3.0 technology capable of video playback at 1080p
Compatible with USB 3.0 enabled computer systems such as Apple Mac & MacBook Pro / Air and HDMI enabled televisions and projectors
USB to HDMI / HDMI Adapter / HDMI Converter / USB to HDMI Adapter / USB to HDMI Converter / External Video Card / USB 3.0 to HDMI
Connect a USB equipped laptop, ultrabook or MacBook to an HDMI-equipped display at your workstation