Q: MacBook Pro won't boot up. Status bar gets to about 75% and shuts down again.

A

Please open the Security & Privacy pane in System Preferences and select the FileVault tab. If the status shown is "Encryption Paused," do whichever of the following applies:

1. If you have a portable Mac (MacBook, MacBook Air, or MacBook Pro), disconnect the power adapter and reconnect it.

2. If you have a desktop Mac (iMac, Mac mini, or Mac Pro) connected to an uninterruptible power supply (UPS) with USB functionality, disconnect the USB cable from the UPS, then restart.

Check the encryption status again. If it's still paused, or if there is no progress after an hour or so, continue.

B

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

Start up in Recovery mode. When the OS X Utilities screen appears, select Disk Utility.

In the Disk Utility window, select the icon of the startup volume from the list on the left. It will be nested below another disk icon. Select Unlock from the File menu. When prompted, enter the login password of a user authorized to unlock the volume, or the alternate decryption key that was generated when you activated FileVault.

Then, from the menu bar, select

          File ▹ Turn Off Encryption

Enter the password again.

You can then restart as usual, if the system is working. Decryption will be completed in the background. It may take several hours, and during that time performance will be reduced.

If you can't turn off encryption in Disk Utility because the menu item is grayed out, you'll have to erase the volume and then restore the data from a backup. Select the Erase tab, and then select

          Mac OS Extended (Journaled)

from the Format menu.

You can then quit to be returned to the main Recovery screen. Follow these instructions if you back up with Time Machine. If you use other backup software, follow its developer's instructions.

Don't erase the volume unless you have at least two complete, independent backups. One is not enough to be safe.

If you want to preserve the data on the startup drive, and it's not already backed up, you must try to back up now, before you do anything else. It may or may not be possible. If you don't care about the data, you can skip this step.

There are several ways to back up a Mac that is not fully functional. You need an external hard drive or other storage device to hold the data.

1. Start up from the Recovery partition, from Internet Recovery, or from a local Time Machine backup volume (option key at startup.) Launch Disk Utility and follow the instructions in this support article, under “Instructions for backing up to an external hard disk via Disk Utility.” The article refers to starting up from a DVD, but the procedure in Recovery mode is the same. You don't need a DVD if you're running OS X 10.7 or later.

If you use FileVault 2, then you must first unlock the startup volume. Select its icon ("Macintosh HD," unless you gave it a different name.) It will be nested below another disk icon, usually with the same name. Select Unlock from the File menu. Enter your login password when prompted.

2. If Method 1 fails because of disk errors, then you may be able to salvage some of your files by copying them in the Finder. If you already have an external drive with OS X installed, start up from it. Otherwise, if you have Internet access, follow the instructions on this page to prepare the external drive and install OS X on it. You'll use the Recovery installer, rather than downloading it from the App Store.

3. If you have access to a working Mac, and both it and the non-working Mac have FireWire or Thunderbolt ports, start the non-working Mac in target disk mode. A Retina MacBook (from 2015 or later) with a USB-C port can also be started in target disk mode and connected to another Mac using a USB cable and an adapter.

Use the working Mac to copy the data to another drive.

This technique won't work with USB (except on a Retina MacBook), Ethernet, Wi-Fi, or Bluetooth. Note that a Retina MacBook Pro (with Thunderbolt) is different from a Retina MacBook, and it can't be connected to another Mac via USB in target disk mode.

4. If the internal drive of the non-working Mac is user-replaceable, remove it and mount it in an external enclosure or drive dock. Use another Mac to copy the data.

I have a similar problem. This issue just started about two weeks ago after running similar updates that were suggested. Although I think it is maybe my graphics card reacting poorly to the update. The computer won't boot up and goes into a reboot loop. WHat is strange is that vertical 1inch wide bars run accross my screen and often intermittient graphics gltiches (like a bad jpg with noise) appear on-screen. I couldn't get it to boot in safe mode at all. I tried all the keyboard shortcuts but they failed to activate. I was interested to get a system test done at the boot-up but with no luck. Just rebooted at least 12 times now and my computer has recovered...but this is now happening every other day now. It is a earl 2011 model MacBook Pro running Yosemite 10.10.5

2.3GHz INtel Core i7, 8GB 1333 MH DDR3 Intel HD Graphics 3000 512 Mb