Have you checked whether your Mac is running the latest version of firmware? For some iMacs, that might bring an unwelcome surprise.
firmware
Many users of iMac 17,1 models – described officially as the iMac Retina 5K 27-inch Late 2015 – […]
According to Apple’s release notes for the Catalina 10.15.3 update, there are only two changes which merit specific […]
Apple has just released the update to bring Catalina to version 10.15.3, together with Security Updates for both […]
Now that Sierra is unsupported, this update no longer marks its older firmware versions as being errors.
The second Security Update for macOS Mojave 10.14.6 (2019-002) was a hefty download considering how few fixes it […]
What they are, how they’ve changed, how to work out what went wrong, and how to deal with them. No Mac should ever experience a single panic.
Diagnostic, Recovery, Safe, Startup Manager, Verbose, reset SMC/NVRAM, firmware restore, Target Disk/Display, Single-User (SUM) in outline.
Why do Macs with T2 chips show a black screen when starting up or updating their firmware, and why so long that it’s scary?
Updates now distinguish between firmware versions for High Sierra and Mojave, and those for Catalina. Recommended for all users.
The Eclectic Light Company 