This has happened before when updating so why not correct this old and serious shortcoming and save unsuspecting customers so much grief?
Imho, maybe the most useful thing forums like this could do is teach users how to safely update OSX. Sometimes I think I've read 4 trillion posts over the years which said something like....
"Help! I updated OSX and then [insert calamity here]!!!!"
Here's a fairly simple method for safely updating OSX.
FIRST: Do you need to update OSX? Do you have a specific reason for doing so? If yes, then....
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1) Copy your entire setup over to a 2nd drive using software like SuperDuper.
2) Update OSX on your main drive by whatever method you prefer.
3) Spend time in your main programs doing whatever you usually do, keeping an eye out for any problems.
4) If you come upon unacceptable problems which you can't fix, or
don't want to be bothered with, copy the backup drive back over
the main drive, thus erasing all problems, and restoring your setup to what it was before the OSX update.
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OR: Copy the main drive over to a backup drive, boot from the backup drive, and update OSX there. The backup drive becomes your testing zone where you can check the update out before making any changes to your main drive.
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Most of the OSX update problems I've seen arise from these faulty assumptions.
1) If it's new it must be better.
2) Free is a good enough reason to grab it.
3) Apple must know what it's doing.
If Apple knew what it's doing it would be Apple that was teaching us the above procedures. As soon as you launched the OSX installer a message something like the above would come up.
If Apple knew what it was doing it would be using this procedure to sell an external drive to those who don't already have one, because having a backup drive is not optional.
If Apple knew what it was doing it would buy SuperDuper and rebrand it as a free Apple product.
If Apple knew what it was doing it would grasp that every time an OSX update fails Apple's brand takes a hit.