Yosemite Won't Boot - Advice Requested

in macOS
Greetings all,

Seeking your advice!

Well, so I woke up this morning and my iMac (running Yosemite) won't boot up.   It gets to the Apple logo and then the machine dies completely.

I can boot from an external drive.   And once I do that I can access files on the internal drive (one that won't boot).

I can't seem to get safe mode or diagnostics to do anything, they start to launch but then machine dies.

I'm confused as to whether I have a bad drive, or a corrupted OSX.   I can access files on the drive that won't boot, so that suggests OSX as the problem.

Any thoughts you may have on this matter will be most appreciated.  Thanks!

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