I Found an Easy Fix for my Recalcitrant Mac Backup Disk

| Dr. Mac's Rants and Raves

Dr. Mac’s Rants & Raves
Episode #156


A couple of weeks ago my Mac displayed an alert telling me that my Time Machine disk was damaged and that I should repair or replace it as soon as possible. The first thing I tried was Disk Utility’s First Aid feature, which has been part of OS X since time immemorial. Sadly, it was unable to fix the damage and suggested I back up my files and erase the disk.

By the way, I’m pleased to report that Disk Utility in OS X 10.11 El Capitan no longer requires you to reboot from another disk in order to run First Aid on your boot disk—yet another reason you might want to consider upgrading to El Capitan if you haven’t already. 

Moving right along, I don’t give up easily, so I tried some third-party disk utilities (which shall remain nameless for reasons that will become apparent shortly), but none could repair the damaged disk.

I was about to throw in the towel and erase the disk, but there was one more thing I wanted try: a Safe Boot. To perform a Safe Boot, first shut down your Mac, wait 10 seconds, and then press the power button. Immediately after the startup tone, press down the Shift key and keep pressing it until you see the gray Apple logo and progress indicator. To reboot in normal mode, restart your Mac again without holding down any keys.

A Safe Boot does a bunch of behind-the-scenes magic. It verifies your startup disk and attempts to repair directory issues if needed. Then, it starts up using only required kernel extensions while preventing all non-Apple Startup and Login Items from opening automatically. Finally, it disables all user-installed fonts and deletes a bunch of system cache files.

I didn’t expect a Safe Boot to fix my damaged external disk, but I figured it was worth a shot before I gave up. Much to my surprise, when I ran Disk Utility’s First Aid after my Safe Boot, it reported that the disk was just peachy and required no repairs.

By the way, OS X 10.11 El Capitan doesn’t give as much feedback as previous versions; to determine if you’ve actually performed a Safe Boot, open the System Information application and click Software in the column on the left as shown here:

Is your boot safe? Only System Information knows…

The moral of the story is that you should try a Safe Boot anytime your Mac misbehaves, even if you don’t expect the Safe Boot to fix anything. This wasn’t the first time a Safe Boot fixed a problem that had stumped me otherwise and it won’t be the last. There’s no downside, so before you start tearing out your hair, give Safe Boot a try.

One last thing: If your iOS device misbehaves, try a reset/restart (similar to a Safe Boot) by pressing and holding the sleep/wake button and the Home button simultaneously, and then releasing them after Apple logo appears on the screen. Like a Safe Boot, there’s no downside to a reset/restart and you’ll be surprised at how often it resolves iDevice issues.

And that’s all he wrote…

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Paul Goodwin

What error messages did Disk Urility give you. My external Time Machine disk crapped out and Disk Utility says it has a bad block and it can’t fix it. It won’trepair, format or partition it.

John F. Braun

I was working on a Safe Boot issue recently and noticed that it can get real persnickety as far as the timing of when you hold down the Shift key.  One option which we’ll discuss in an upcoming show is how to set the boot mode via the Terminal and the nvram command.  Also, once the initial progress bar is done, and you’re on the login screen, you should see “Safe Boot” in red letters on the top of the screen, though sadly this goes away once you get into the OS proper.


Over the years my Mac(s) have given me that error. Each time I tried repairing it and when that failed I’d sigh and nuke the backup disk and start over. (I run a couple of levels of backup so it was never the ONLY one). Next time I see the error, and there will be a next time, I’ll try the Safe Boot trick you described.

Bob LeVitus

Paul: I didn’t grab a screen shot (I usually do) so I don’t know the exact wording of the error… Sorry. 

John: I know about seeing Safe Boot at the login screen… But you used to see it in the menu bar when the Finder appeared…. That’s what I meant about it being easier to know if you were booted safely in previous versions of OS X.

Geoduck: It worked for me much to my surprise. From now on I’m going to try a Safe Boot before I try any actual troubleshooting for anything. It can’t hurt and I’ve decided it can even help when you have no faith it’ll help.

Paul Goodwin

I tried Disk Utility when in Safe Boot mode, and it behaved the same. When I try to erase or partition, it just can’t do anything with the disk. Something has apparently really gone bad on the disk. Oh well.


This was a huge help.  I’ve had to reformat and start over on my Time Machine disk a couple of times over vague error that wouldn’t allow it to backup anymore and Disk Utility being unable to fix it.  Booted into Safe Mode and ran First Aid and everything kosher again.  Thanks!

Bob LeVitus

TitanTiger: That’s precisely the point I was trying to make - you never know what a Safe Boot will fix, so it’s almost always worth a try when you’ve got issues. Glad to hear it helped you.

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