2020 security update issue

Are you running any VPN, Anti-Virus, or Cleaning apps?

How much free space on your Hard Drive?

Repair a storage device in Disk Utility on Mac

Disk Utility can fix certain disk problems—for example, multiple apps quit unexpectedly, a file is corrupted, an external device doesn’t work properly, or your computer won’t start up. Disk Utility can’t detect or repair all problems that a disk may have.

If you run First Aid on a disk, Disk Utility checks the partition maps on the disk and performs some additional checks, and then checks each volume. If you run First Aid on a volume, Disk Utility verifies all the contents of that volume only.

1. In the Disk Utility app  on your Mac, choose View > Show All Devices.
2. Note: If you’re checking your startup disk or startup volume, restart your computer in macOS Recovery, select Disk Utility in the macOS Utilities window, then click Continue. If you check your startup volume (Macintosh HD), make sure you also check your data volume (Macintosh HD - Data).
3. 
   
4. In the sidebar, select a disk or volume, then click the First Aid button .
5. If Disk Utility tells you the disk is about to fail, back up your data and replace the disk—you can’t repair it. Otherwise, continue to the next step.
6. Click Run, then click Continue.
7. If Disk Utility reports that the disk appears to be OK or has been repaired, you’re done. You can click Show Details to see more information about the repairs. Otherwise, you may need to do one of the following.

• • If Disk Utility reports “overlapped extent allocation” errors, two or more files occupy the same space on your disk, and at least one of them is likely to be corrupted. You need to check each file in the list of affected files. Most of the files in the list have aliases in a DamagedFiles folder at the top level of your disk.
    • If you can replace a file or re-create it, delete it.
    • If it contains information you need, open it and examine its data to make sure it hasn’t been corrupted.
  • If Disk Utility can’t repair your disk, or you receive a report that the First Aid process failed, try to repair the disk or partition again. If that doesn’t work, back up as much of your data as possible, reformat the disk, reinstall macOS, then restore your backed-up data.

If your Mac has a Fusion Drive and you see a flashing question mark or alert, see the troubleshooting section of the Apple Support article About Fusion Drive, a storage option for some Mac computers.

If you continue to have problems with your disk or it can’t be repaired, it may be physically damaged and need to be replaced. For information about servicing your Mac, see Find out how to service or repair your Mac.

Get the standalone update & install it in Safe Mode...

https://support.apple.com/kb/DL2046?locale=en_US

Safe Boot from the HD, (holding Shift key down at startup).

BDAqua's troubleshoot may be helpful, but my bet would be that Apple has released another problematic security update. If you do a search of prior security updates on the Apple Community Support you'll see a large number of users who have had serious issues with installing these updates. Also check for issues with this update. There's another post by a user with an install issue. There are also some excellent workarounds posted on prior recent updates on the Support pages, so I would reference those. I would never try an install of a security update upon release. First, wait a few weeks, check the Apple Community support pages on the update, see if there are multiple users that are reporting issues and possibly providing solutions, then make your own decision about whether to install it. Some of the bad installs have been really disastrous necessitating complete OS re-installs, which is what happened to me on a prior security update. It's a disgrace that Apple doesn't make these installs bullet-proof anymore.

Good points, last 3 Security updates gave me fits, no idea as to the cause but found out if I cloned to a new drive, applied the update to that drive, then cloned back I was good... & not a thing wrong with the 1st drive... sector scan & all!?