I just installed Sierra 10.12.6 (using a fresh download from the app store, and then created a USB installer) on an external G-Drive connected to my 5K 2017 iMac. Installation went fine. When I attempt to playback iTunes media, the picture is all pixelated and broken, albeit the sound is smooth and clear. YouTube video is fine. I tried updating iTunes and that didn't correct the problem. When I boot into Mojave, the iTunes playback is normal.
Any ideas on what the problem could be or how to fix the pixelation problem?
This is link shows a picture of the problem although not identical.
Any ideas on what the problem could be or how to fix the pixelation problem?
This is link shows a picture of the problem although not identical.
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