White Screen Hang Mac Pro
Already tried NVRAM reset?
I've been surprised by how many problems this &quot;fixes&quot;:
https://support.apple.com/en-us/HT204063

Turn on your Mac.
Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
Hold these keys until the computer restarts and you hear the startup sound for a second time.
Release the keys.