iMac chimes but no apple, black screen for 4 minutes, then login scree
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?
Is this a good question?