Q: 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!

A: 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.
