Chosen Solution
After I had the iMac unplugged from power for about a week, its startup time changed from about one minute to 1-2 hours. Start system is a Samsung SSD via USB (worked fine for many months). I have reset PRAM and VRAM, started with the SSD as the only connected device, built a new start system 10.13.6 on another SSD, but never got a startup time shorter than 75 minutes. Once the Apple shows up, everything progresses normally, and the iMac is usable after another 40-80 seconds. Thank you for any suggestion, I’m totally out of my depth for this one.
Try booting from an external drive running EL Capitan and disconnect your other externals devices for now. We are experiencing multiple issues with third party drives and High Sierra and Mojave. Also with MicroSoft Office 2011.
I’m sorry I missed the diagnostic error code you posted VFD003 which is a display sensor error. What is happening is your system is in Safe Mode to protect it from overheating. Reference: Apple Diagnostics: Reference codes So at this point I think you’ll need to bring in your system to get the display replaced.
