iMac won't start up

[cliffy1]

How do I start a iMac that keeps shutting down half way through start up?

 

[grahamperrin]

What exactly happens before the shutdown?

Using Apple Diagnostics - Apple Support

OS X El Capitan: Ways to start up your Mac

How to start up your Mac in single-user or verbose mode - Apple Support

 

[cliffy1]

The apple symbol comes up and the slider window starts to move acros. It gets about 1/4 of the way and the machine shuts down.

 

[grahamperrin]

OK. Now boot in verbose mode and take a note, or a photograph, of the last thing that is on screen before the unexpected shut down.

 

[cliffy1]

OK. Now boot in verbose mode and take a note, or a photograph, of the last thing that is on screen before the unexpected shut down.

I ran Disk repair. It said the disk cannot be repaired. I think it might be toast.

 

[grahamperrin]

OK. Now the shutdown may be explained.

There is a limit to the number of times that the operating system will automatically run fsck_hfs when an HFS Plus startup volume is found to be DIRTY.

When that limit is reached, no further attempt will be made to repair the file system inconsistency (until the next boot/startup) so to me, it makes sense to automatically shut down.

If you are extremely lucky, further attempts will successfully repair the inconsistencies. Alternatively, further attempts may worsen the situation.

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[cliffy1]

I took a picture of my screen after starting with Command-option-R, if this helps:

 

[Fishrrman]

You need to try booting from an "external source".

Otherwise, you're going to keep going around in circles...

 

[maflynn]

I ran Disk repair. It said the disk cannot be repaired. I think it might be toast.

I took a picture of my screen after starting with Command-option-R, if this helps:

You answered your question by saying the drive is toast. No amount of debugging, will help you. Unfortunately the drive seems to have failed.

If its in warranty, call apple, if you feel brave open it up yourself, but its not for the faint of heart. Its a risky proposition especially if you have a fairly new iMac. If not, then call an authorized repair shop (or apple) to get it repaired

 

[grahamperrin]

I took a picture of my screen after starting with Command-option-R, if this helps:

Yes and no, because it shows the automated HFS Plus file system check (fsck_hfs) only part way through its routine, checking catalog file … without showing the point at which a problem occurs.

Hint: if that's a row of rrr… in your photograph, then you can safely release the R key

More importantly, if you want Recovery OS then it's Command-R (not Option-Command-R).

Back to my first link in post 2 above, try Apple Diagnostics. Do the results of a diagnostic routine mention anything about the disk?

Anything about 'S.M.A.R.T.' or 'SMART' maybe?

(You might, alternatively, get S.M.A.R.T. information for the disk (that contains the file system) with Disk Utility in Recovery OS, but I should take this opportunity to run Apple Diagnostics.)