Black screen and loud fan!
When starting my iMac it will not boot up and the fan comes on loud
A few things could be going on here with this, to be honest :
1) Have you tried reseting the PRAM /SMC while its booting?
2) have you made a backup of the unit recently just in case of # 3
3) Could be HDD failure that it is not picking up the boot drive hence the fans are going nuts.
Personally id start with one and then go from there if you do end up opening up the unit make sure to reseat and double check all your internal connections before replacing parts ie.HDD. Also make sure that you give the fans and the heat sinks a good cleaning with compressed air. Also be vary careful when you do open the unit that you DON'T rip off the display cables only open it with about two or three inches of clearance just enough to disconnect the cables.
When you decide to open the unit unplug the unit from the wall and let it sit for about 20 mins so that any residual power still in the power supply has time to bleed off. Otherwise you will be receiving a nice jolt from it if you decide to stick your hands in the right off the batt, but you also stand the chance of completely destroying your unit with an ESD accident. Lets stay safe K
This is an odd one ...
Replacing the HD in the 21.5” models is not a big deal as Apple uses the 2.5” drives SMART services. In the 27” series has a custom 3.5” HD drive. So in it’s self that doesn’t bother me.
Now the question, did you have someone put in (or out) the custom blade SSD on the back side of the logic board? Why I ask here is I suspect that may have been the cause of the discharging of the PRAM battery (prematurely).
So here’s the chain of events the system being powered (AC) gets enough power to hold its settings (on or off). But, this time you disconnected the system boxing it up for the storm. Now the PRAM battery kicks in ... after things settle down you unpack the system and bang! Nothing happens other than the high speed fan as the PRAM battery is dead
So ... if that’s correct then you’ll need to open the system up and remove the 3rd party blade SSD and replace the battery which is on the back side of the logic board as seen here marked in blue: