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!
To attempt to back it up you could try starting in Target Disk mode &amp; 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.