iMac will not boot

iMac, running Sierra.  iMac won't start up.  Progress bar goes to about 80-90%, freezes, then computer shuts down.  Tried SMC and nvRAM reset, no luck. Went into Disk Utilities and ran First Aid, unable to repair: invalid sibling link (4,73255).

Any suggestions or is my disk probably done?  Thank you
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David FavorLinux/LXD/WordPress/Hosting SavantCommented:
When I have problems like this, I connect an external disk dock + boot from the external disk.

Then run you disk repair.

If your disk can't be repaired, then pull off data you require, then see if a format + reinstall on the problem disk might work.
D219Author Commented:
Thank you, sorry but above my ability.  I have an external hd attached to iMac buy I do not know how to boot from it.  Do I need some sort of disk from Apple?
D219Author Commented:
Trying to quickly educate myself.  If I purchase an external disk dock, I will need an external disk - is that from Apple?  Or can I use my functioning MacBook Pro (Sierra 10.12.6) in any way?  Thank you
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Owen RubinConsultantCommented:
What year is your iMac? Does it have a DVD drive? And do you still have the DVD that came with the machine? You could try booting from the DVD if yes to these questions. Just hold down C when starting your Mac. On that DVD should be the ability to run Disk Utilty to try and repair your machine.
web_trackerComputer Service TechnicianCommented:
You cant really boot off of a working disc or another computer. Best option would to boot off a working Sierra operating disc as suggested by Owen. iF you cant repair the operating system you may need to pull out the disc from this machine and put it into drive doc and connect it to another mac to recover your data off the drive. Then once you have your data backed up you can do a fresh install.
D219Author Commented:
Thanks to all.  I ended up just erasing the HD and reinstalling everything using Time Machine, took about 6 hours.

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