iMac shuts down during startup

I'm experiencing a shutdown of an iMac during startup, when the progression bar almost halfway.

I tried, with no success: - starting up from the iMac recovery partition and checking the HD with Disk Utility (rebuilt permission, verified the HD) - Safe Boot - Fsck

Before attempting to reinstall Mac OS I think I need to backup the iMac first.

Any other check suggested before trying to re-install Mac OS?

Thanks for any insight!

Solution 1:

To attempt to back it up you could try starting in Target Disk mode & connect it to another Mac.

It is potentially safe - as you've checked the disk already with no errors - to simply overlay a new OS on top of the existing one without disturbing any other data. Run 'Install macOS' from the Recovery partition.