Yes it happens on an external display. I tried it with a live Debian USB. It took a moment but I got similar glitches right after the graphics mode selection.
... actually? Yes they continue. Both SSDs are fine according to HD Sentinel, so unless I zapped the logic board with an errant static discharge I seriously don't know what even is going on.
I can think of two factors that could have caused it. #1 During HDD replacement, you may have interacted with the screen hinge bracket. You may boot up your Mac and move the hinge / display cable package or screen a bit to see if the graphic artifact changes. If it doesn’t change most likely it’s the GPU chip on the mainboard. I have also the MBP 2010 (17‘). My GPU chip was resoldered once after 5 years. The first repair worked for two years. Than it happened again and I bought a refurb mainboard that still works. There are several places out there that do both, resolver and replace mainboards.
