Sometimes updates go wrong. Like this time for me, anyway. I wasn’t worried because I had a backup that was less than 24 hours old, but I was angry. “Stupid computer! You update now!”
First, it’s important to realize that your Mac may just lie to you, possibly because it hates you. I don’t know. Who knows what motivates a computer? Mine repeatedly insisted that the update “was corrupted or may have been tampered with”. That seemed unlikely, particularly after I downloaded it five times.
But you don’t have to get these updates from the Mac App Store. They’re also available as stand-alone installers from Apple’s download page. In my case that didn’t work either.
Time to check the disk. To run Disk Utility in full repair mode on your Mac, restart and hold down command-R. Then select Disk Utility from the list of options that appear.
Oh, and pro tip on that: if you’re using a Bluetooth keyboard with a MacBook, hold down command-R on the MacBook, not the Bluetooth keyboard.
Once I repaired the disk, the update worked fine. Now I can enjoy not being able to access the Users folder.