Why does my iMac crash on boot and display this weirdness?
crwdns2934249:0crwdne2934249:0:
Most of the time my iMac 2017 21.5” crashes during boot. The normal white apple appears and the loader bar starts progressing, but then the screen blinks to black. I think the keyboard capslock light can be turned on/off, but it makes no sounds or reactions. I have to hold the power button until I hear a very quiet fan power down. Then I can attempt to boot it up again. Holding down shift will always succeed at booting into Safe Mode, and that’s the only way I’ve been using this iMac for a long time now, which is quite sluggish. Sometimes it can boot normally without having to go into Safe Mode and I can log in, but often it will crash to a black screen during use at some point.
When it fails during boot, occasionally I see some odd things on the screen for a split second before it blinks to black. Here’s one colorful example:
[image|2358977]
It doesn’t happen the same way each time.
Here’s another more detailed example I caught on video. It feels like it takes a long time to boot. Where you see this picture’s progress bar, that’s more than 3.5 minutes into boot.
[image|2358980]
Then it blinks to all black for 1 second, then it displays this distorted progress bar and white apple for 2-3 seconds, then it blinks to black again.
[image|2358981]
After 13 seconds of black, it then displays this alert message.
[image|2358982]
This was happening while running macOS Mojave, if that makes any difference. I later updated to Catalina, but I still have boot problems.
[image|2358985]
This video weirdness makes me suspect there is something wrong with the video hardware. Does that sound likely, and is there a way to verify that? Is there actually a replaceable video card in this iMac?