user account login since update to OS x 10.11.6

I have the same issue as described in the original question and could use guidance for Step 1.

I use Time Machine, but my last backup was 2 mos ago. (I know, I know. I thought security updates were not a big deal. Lesson learned.) I'm hoping I can backup now before proceeding with other steps.

I was able to log in as Guest and create a new admin account. Any help is much appreciated.

Software update just prompted me to update to 10.11.6 with a security patch. It went fine on the Mac Pro, but it bricked my main user profile on my MacBook Pro. Tried the usual restart in safe boot, but it won't get past the first screen and actually load the desktop.

Oddly, the MacBook Pro shows up on the network, and I'm preparing to back up 400+Gb of files to the MacPro. We'll see if that works. Umm, no, it doesn't Says I don't have permission to read them. Going back to copy directory by directory. I'm guessing it won't copy Library files...

I was able to log into the Guest account, as well as another admin account on the MBP. However, it made me setup my Apple ID again, with a confusing set of instructions, which <knock on wood> don't mess up my other devices.

My latest backup on the MBP is about a month old, and I'm hoping that a network copy will work.

Otherwise, I may try to install a fresh copy of OS X 10.11 on the other internal drive in the MBP and go from there. (It has an SSD and the old HD in the optical slot.

Do you have another computer? Does it see your machine across the network?

If it's an older machine, Target Disk Mode may work... google that. It only works with FireWire and Thunderbolt machines, and the cable ends need to match. That could be problematic with Thunderbolt, as I have never seen a double ended Thunderbolt cable. Thunderbolt to USB, yes.

Removing the launch store caches using the process described worked. After backing up, I deleted the files, and the MacBook Pro, which was hung in between user log-in on my normal profile, and the desktop, proceeded to boot through the process of re-creating a user password (which now cannot be the same as the Apple ID password), and asking for the old KeyChain password. This double-factor authentication may be causing some of the issues. I was applying a security patch to OS X 10.11.6, not going to a new OS X version. It worked fine on my desktop machine, but failed on the laptop. Seems to work now, though. Thanks to the OP.

1. What backups do you have?
2. Do you have more than one user account?

The advanced shortcut below, if it works, may save you the need for a clean reinstallation of OS X. Take these steps only if you feel confident that you can follow the instructions.

1. If you don't already have a current backup of all data, please back up now. There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

2. Disconnect all external storage devices.

3. Start up in Recovery mode. Select a language, if prompted. The OS X Utilities screen will appear. One of the options is to install OS X. That's not what you're going to do.

4. This step is only necessary if you use FileVault 2. If you don't know what FileVault is, you're not using it. Go to the next step. Otherwise, launch Disk Utility, then select the icon of the FileVault volume ("Macintosh HD," unless you gave it a different name.) It will be nested below another drive icon. Select Unlock from the File menu and enter your login password when prompted. Then quit Disk Utility to be returned to the main screen.

5. Select Get Help Online. Safari will launch. While in Recovery, you'll have no access to your bookmarks, but you won't need them. Load this web page.

6. Triple-click anywhere in the line below to select it:

for f in /V*/*/var/folders/*/*/0/*.csstore; do mv $f ${f%/*}/../T/; done

Copy the selected text to the Clipboard by pressing the key combination command-C.

7. Quit Safari. You'll be returned to the OS X Utilities screen.

From the menu bar, select

Utilities ▹ Terminal

The Terminal application will launch. Paste into the Terminal window by pressing the key combination command-V.

Wait for a new line ending in a hash sign (#) to appear. Quit Terminal to be returned to the main screen.

8. Select

 ▹ Restart

from the menu bar.

This worked for me. Could log into other user accounts. But not main User (admin) account. You are a life saver!

Like the others here, El Capitan update locked me out with an endless spinning gear after entering password. Replaced the OS via Recovery partition (a 6-hour download here + 45-min install), was no help. Linc's solution worked perfectly, the man is a true genius! Thank you, Linc.