Processes crashing during startup with "too many corpses" error.

Assuming that your post in reply to a post of mine in this thread is referring to the same issue, and you're unwilling to revert back to Yosemite (and that you have no Time Machine backup), your only option may be install El Capitan from another drive.

Unless you've still got the Beta 1 installer somewhere, you're going to need another Mac to download it because your AppleID isn't being recognised from Recovery Mode. You can then either put your mac into Target Mode or make a bootable installer on a USB.

If you don't have access to another Mac then the only other option would be to see if Internet Recovery Mode will let you install Yosemite to an external drive and download it from there (it won't let you install to a 2nd partition on the internal drive because there's already a later version of OS X there).

Let me know which of the above is applicable to you.

You're not the only one whose AppleID isn't being recognised in Recovery Mode either. You're the third I've heard of since yesterday. Fwiw, there'd be no point in /Library/Applications Support/iCloud/Accounts on your El Capitan partition. You could, however, try looking for and removing a similar folder from the RecoveryHD partition you're able to boot into.

That makes sense. I'll have to research Target Mode (never used that before) and see what I can do.

My main question here, however, was what those errors mean and what's the likely cause. If we knew the cause then it would help to narrow down the potential solutions. In this case, I'm assuming an install is necessary but maybe it was something else. I'm just trying to understand so I'm not thowing solutions blindly at a problem. 😉

Thanks for the help!

There's nothing on that error message - I've searched. The only reference to it is verbose failed bootups (like yours) that ended up in the user having to reinstall in one of the ways I describe above.

On a related note though, it's possible that the Recovery partition isn't accepting your password becausing auditing (which causes passwords not to be properly accepted even when they're right) got triggered somehow. It's a bit of a shot in the dark but, if you want to try it you'll need to copy-paste the following into Terminal launched from Recovery Mode:

rm /var/audit/current

If it returns an error then it was never the problem.