Chosen Solution
I replaced an old HDD with a 1TB WD Blue SSD and after restarting I first thought I had not connected the display properly as it chimed, but was black screen. I disconnected and reconnected, still the same. I reset the SMC and PRAM and after leaving the machine for about 5 minutes it suddenly produced a login screen. It then seemed to work normally. Restarting after updating everything possible to update produced the same 5 minute wait to login. Trying to boot from a USB installer produces the same 5 minute wait before showing the choice of boot device Connecting another monitor gives the same black screen on the second monitor. No output until after the login screen appears. Putting the old hard drive back in produces the same symptoms. Holding D and doing a hardware test gives a clean bill of health (LOL) I’ve done dozens of these before with no problems and I’ve triple checked all the cable connections I can see to check. How can I diagnose this fault?
Can you hold Command + V keys on power-up and see if it boots under verbose and if not where does it get stuck? Also, you can try to boot to safe mode: Prompt your Mac to restart. If your Mac is currently on, you’ll need to begin restarting it before you can access Safe ModePress and hold the ⇧ Shift key. As soon as your Mac begins restarting, hold down the ⇧ Shift key and don’t let goWait for the login screen to appearRelease the ⇧ Shift keyLog into your Mac
I would check the battery to make sure its good and with the battery connected check the voltage at the carrier. Your system is loosing its NVRAM (PRAM) settings. Which is why it takes a bit of time to run through the config POST which only happens when you alter the RAM setup (or when the battery is bad) Clearly this is a logic board issue with the power logic. You’ll need to get the schematics to trace out the power rails.
Go to System Preferences > Startup disk > Select your boot disk and then press the restart button. your iMac doesn’t currently have a default hard drive selected that’s why it’s taking ages to detect it. if it’s currently taking a while to start up hold down the option key and select your drive.
