iMac stop working and shutdown

We were not there to look over your shoulder to see all the steps you took so we really have no idea. However unless you followed the same procedure in Use an external SSD as your startup disk … - Apple Community

it will likely fail.

My take though is the computer is told old to try and fix. Personally I’d just replace the thing as it’s obsolete and considered vintage. This means it cannot run current versions of Mac OS and parts are largely unavailable. In short, it just not worth the trouble.

mirkomk wrote:

Can you help me identify the problem?

Sadly, no, I cannot!

Many thanks for any suggestion or idea to identify and solve the issue.

However, it seems in all likelihood that you have all your data backed up so, if this computer was mine, I would start it in Recovery Mode. Details here.

Then, I would use the Disk Utility to completely erase my data. Once done, I'd then Install a fresh version of the operating system. It should take little more than half an hour in my experience. You should then have a pristine canvas upon which to paint - starting from "Hello"!

Then reassess how the iMac is performing and post here to let us know!

How does that sound?

The report indicates:

" Low disk space - This computer is running critically low on free hard drive space."

This means you need to offload large libraries such as your photos library, music library and movie library to an external HD!!!!!!!!!!!!!!!!!!!! This is likely the cause of your problem. As a general rule Mac OS need a minimum of 10-15% of available space to operate, your system does not have that. Apple explains how to move the photos library to an External Hard Disk, the process is the same for music and movies. Please refer to: Move your Photos library to save space on your Mac

mirkomk wrote:

Which macOS are you suggesting to reinstall? The internal macOS or the macOS installed on the external SSD?

Hi, apologies for the delay in getting back to you. On the assumption that you HAVE a back-up of all your data, I suggest disconnecting all extraneous SSDs from your main computer. You'll then have no other option than what I first suggested - use macOS Recovery, the built-in recovery system on your Mac, to erase and reinstall macOS.

I suggest that you follow the procedure here:- Erase and reinstall macOS – Apple Support (UK)

Once you have done so, and your computer restarts, before doing anything else just let it run - and play around - without installing anything else. Try to judge if your machine has got over the problems you were experiencing when you posted your question.

Then let us know, please.

Please download and run Etrecheck.  The free version is sufficient.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and after clicking on the Reply button use the Additional Text button to paste the report in your reply.

Check Etrecheck's Privacy settings and make sure the checkbox for full disk access before running:

Then we can examine the report and see if we can determine the cause of the problem.

Please download and run Etrecheck.  The free version is sufficient.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and after clicking on the Reply button use the Additional Text button to paste the report in your reply.

Check Etrecheck's Privacy settings and make sure the checkbox for full disk access before running:

Then we can examine the report and see if we can determine the cause of the problem.

Are you able to reboot immediately after a shutdown?

If not try the following:

1 - unplug the power cord from the back of your iMac for about 15 seconds.

2 - plug it back in and try to reboot.

If the above works then you are probably experiencing a failing power supply. I know I had the same issue with a 2015 iMac.

You're running very low on free space:

Free: 27.64 GB

That could contribute to your crashing. Is the free space the same on the external SSD that you're booting from the same or about the same?

Also unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint.  Read these articles:  Pubic VPN's are anything but public and Security Risks: The Dangers of Using Free VPNs (eccu.edu).  \

I'd free tip an additional 80-100GB of free space and see if the problem persists.

Also I recommend you uninstall the VPN according to the developers instructions

Erase and reinstall macOS – Apple Support (UK)

Extra detail in this link too!

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