I want OS 10.12.n back

I'm afraid that the routine for fixing a locked disk if the disk has not failed, requires using Terminal commands.

If you feel comfortable trying to use Terminal then here is what you can try:

Fix Permissions for a Locked Drive

Open Terminal in the Utilities' folder. Copy and paste the following commands at the prompt. Press RETURN after each. The first command will prompt for you to enter your admin password. It will not be echoed to the screen. After entering your password press RETURN again.

sudo chflags 0 /Volumes/*

sudo chmod a+rx /Volumes/*

killall Finder

Now, if this doesn't do the trick you will need to move on and try this:

I looked first hand at this problem and the following commands unlocked and formatted the disk, although Disk Utility could not.

$ diskutil list

Should show something like this:

/dev/disk0

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *121.3 GB disk0

1: EFI 209.7 MB disk0s1

2: Apple_HFS Macintosh HD 120.5 GB disk0s2

3: Apple_Boot Recovery HD 650.0 MB disk0s3

/dev/disk1

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *64.0 GB disk1

1: EFI 209.7 MB disk1s1

2: Apple_CoreStorage 63.2 GB disk1s2

3: Apple_Boot Recovery HD 650.0 MB disk1s3

Then format the disk in question. In this case, disk1.

$ diskutil eraseDisk JHFS+ name disk1

You should see this:

Started erase on disk1

Unmounting disk

Creating the partition map

Waiting for the disks to reappear

Formatting disk1s2 as Mac OS Extended (Journaled) with name name

Initialized /dev/rdisk1s2 as a 59 GB HFS Plus volume with a 8192k journal

Mounting disk

Finished erase on disk1

The following web pages helped me figure this out and might be of use to somebody else.

https://discussions.apple.com/thread/4091299?start=0&tstart=0

http://derflounder.wordpress.com/2011/11/23/using-the-command-line-to-unlock-or- decrypt-your-filevault-2-encrypted-boot-drive/

Note that the above was not written by me but I have used it and it does work. But if you don't feel comfortable with it then skip it. Take your machine in for repair. You may find that the HDD needs replacing.

If your model works with Net Restore and you cannot restore a backup:

Clean Install of El Capitan or Later on a Clean Disk

1. Restart the computer. Immediately after the chime hold down the Command-Option-Rkeys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Reinstall OS X and click on the Continue button.

This should install the version of OS X that was pre-installed when the computer was new.

Erase the disk drive and then restore the backup made prior to the upgrade.

Restore from the backup you created prior to upgrading.