iMac shuts down randomly for no apparent reason

Ah yes, I've done more testing on mine this weekend and have determined that any drive plugged into either one of two USB ports (although it has its own power source) will result in a completely dead/not responding/shutdown??? iMac 100% of the time after letting it go to sleep. When there is no drive plugged in, the dead/not responding syndrome will be there about 50% of the time. So, I'm guessing logic board (because of the USB ports)? Shutdown causes are still 0 and 5. It will respond to the power button if I unplug/plug it back in; if I don't, it does not respond to wired/wireless keyboards or mice. But, technically, it may not think it's a shutdown because I get this:

"Unclean" shutdown! Really. If it doesn't respond and displays nothing but a black screen, it sure looks like it's shut down to me...

I had a 2011 27" iMac that ran flawlessly for 2 years and then began freezing up requiring power resets. I took it back and forth to my local Apple store multiple times and each time different parts were replaced, graphics card, memory, hdd, logic board and so on. The 5th time I went to pick it up it wouldn't power on at all. After talking to the store manager they decided to write off the hardware as unrepairable and ordered me a new 2013 comparable model replacement machine. While I invested a bunch of time trying to have the machine repaired I was satisfied with the final result.

Apple support does have an internal program to allow for this. If your machine is still on Apple Care support as mine was then I suggest you ask to speak to the store manager and plead your case. Be nice and explain that you have invested more than enough of your own personal time and the inconvenience of multiple trips to the store without a satisfactory repair.

I just had my 2013 iMac in for the 3TB Fusion drive replacement which was done on a warranty replacement program. Details here. https://www.apple.com/support/imac-harddrive-3tb/

Hopefully they'll be able to do something similar for you and give you a replacement iMac.

One of MacKeeper's endearing features is its ability to delete original photographs in your Photos or iPhoto Library. That is just one example of its irreversibly destructive actions. As I wrote MacKeeper is only one example.

How would I reverse this overall corruption?

You cannot. There is no way to undo that, other than to restore your iPhoto Library from a backup created prior to the onset of corruption. Reinstalling OS X will have no effect, other than to restored corrupted operating system components themselves. It does not remove any third party modifications.

Can I save my photo and video files and just start over with a new computer?

Yes. You can save the present state of your Mac's storage by using Time Machine: Use Time Machine to back up or restore your Mac - Apple Support.

Thanks for that link mikestan56. I have not read every single post because so many of them understandably just want to relate their own tales of misery and / or just rant about the issue which makes it a challenge to find anything useful.

However, I did find this one ("Link to this post" doesn't seem to work for me):

blinkypete wrote:

An Additional Update:

Since obtaining my system from repair (i.e., power supply and power cord replacement) over a week ago, I have not experienced an intermittent shutdown.

I suggest obtaining the shutdown code that is associated with an intermittent shutdown. In my situation, as soon as the Genius Bar representative had this code, he was able to make a diagnosis and propose a repair solution. Granted, I am still under Applecare and that may have been a factor in his little hesitation to repair the system. Prior to obtaining this code, the Genius Bar was unable to replicate the issue.

Finally, although I may or not be in the clear and that each situation has similarities and differences, I'm thus far satisfied with how Apple, specifically Matt (Genius Bar Representative) at the Northbrook, IL location, handled my system's issue.

As I wrote a long time ago that shutdown code (zero) indicates a power interruption that points to a power cord as one obvious component, and the power supply, another. You got a new power cord but blinkypete replaced both, and since he hasn't posted anything subsequent to the above I can only assume his problems have been resolved to his satisfaction.

The point is that someone has to do something to advance troubleshooting toward an ultimate solution. So far, Apple hasn't done anything productive for you toward that end. A Mac is a complicated piece of hardware, and something is obviously defective with it. Whatever that is needs to be replaced. Until someone starts replacing some parts, even if you have to pay for it, you're not going to get anywhere.

Previous shutdown cause: 0

That code will appear if the power to your Mac is suddenly interrupted. You can confirm that yourself by disconnecting its power cord from the wall receptacle while the Mac is operating.

The causes can be due to any one of the following

1. a defective power cord, which might not be evident by inspecting its exterior
2. an intermittent connection between the power cord and the Mac's power receptacle
3. a faulty (household / office) power receptacle
4. a faulty power strip / surge suppressor
5. a defective Mac power supply
6. a crack or similar break in the Mac's circuit board

The most likely cause given the history you reported is #1. You can substitute any standard electronic device power cord that fits.

What could possibly be the issue with this machine after all the testing and interventions that have been done?

A hardware fault. Don't waste your time with any "malware detection software", "antivirus scans", or any similarly categorized garbage.

Open Console - it's in your Mac's Utilities folder. To determine the shutdown cause, examine system.log for "shutdown cause". The log entry will be "Previous shutdown cause:" followed by a numeric code. What is that code?

Is there any way to access system logs further back in time.

Yes. Archived logs are named system.log.0.gz and so forth. They can be opened and examined the same way as system.log, which is normally archived daily at midnight system time.

Power interruption is causing the random shutdowns. Apple's support "geniuses" should have been able to determine that within minutes of accepting it for repair the first time, much less three or four return visits.

If it is a defective Mac power supply or a crack in Mac circuit board wouldn't that have been identified in a hardware assessment?

No, other than to identify the cause of the shutdown being a power interruption. The power supply itself might be defective, but all the logic board (the SMC to be specific) is able to determine and report is an unexpected loss of power. The same shutdown cause would occur if there were a break in any circuit between the iMac's power source and the logic board. The break might be microscopic and difficult to find. It might also change during physical movement or thermal expansion, which can be induced by bending or stressing various connectors for example.

It could also be brought on by a short anywhere in the power supply, perhaps an intermittent one which could have been caused by an object becoming dislodged from the back of the iMac. 27 inch iMacs such as yours have been known to have that bracket fail internally. It's possible an internal fastener became loose and found itself conducting an electrical path where one ought not to be. Loose things or otherwise conductive debris inside a computer will cause random failures that can be difficult to find.

As you see there can be quite a few potential causes. A problem such as that might require board-level repair, which has become a lost art. Most computer repair shops would prefer to substitute components, such as the power supply (which is not expensive) or the logic board (which is), observe the results, and stick you with the bill.

At what point was someone going to discuss or target the only item that powers on or off a machine? A power controller takes various inputs to decide when to power cycle. It even determines when the CPU is permitted to execute. One controller input is the front panel power button. Controller even acts differently depending on how long that button is pressed. Another input comes from the OS - when the OS asks for a power off. If the OS asks for a sudden power off, then the OS also says so in logs. Another input is monitor circuits in the power supply. If those demand a power off, then the OS and its logs know nothing about it.

How to find an informed tech? He knows about this hardware and how to measure it.

What diagnostics did Apple techs run? A processor called the power controller has no hardware diagnostics. CPU cannot monitor or obtain status information. CPU is simply told by this processor when to and not to execute. What diagnostic could Apple (software) techs run to diagnosis a part of hardware that software cannot see? IOW get an informed tech.

Power cord and surge protector clearly are irrelevant. Your Apple must work just fine even when incandescent bulbs dim to 40% intensity. How did a power cord or power strip cause voltage to drop that much - without also getting hot and dangerous?

Your problem clearly requires someone who knows hardware. Who does not blame things without any reason to. And has simple layman tools to view what that hardware is doing. Again, find a tech who knows how electricity works. That is a first problem. Power off solution comes later.