My friend asked me to look at her early 2011 Mac Mini with OS X 10.8.3. She said that:

  • she installed software updates
  • she couldn't remember which updates were installed
  • when the Mac restarted, it hung at the Apple logo with the spinning gear wheel.

I used Disk Utility to check permissions and the disk. Both checks found a few errors, all of which were repaired. This did not help; after I rebooted, it hung at the same screen.

Several resets of the PRAM did not help.


A safe boot succeeded, the desktop appeared. I re-checked permissions and the disk; no errors.

Then a normal boot succeeded – this surprised me.


Can anyone guess what was wrong and what was fixed?

Since it's not my Mac, and I don't know how it's really used, I can't guess.

I see Apple article HT1564, Mac OS X: What is Safe Boot, Safe Mode?, I'd like to understand more.

  • 2
    Starting in safe mode clears some of the system caches, I believe. So if one was corrupted, it would have been re-cached.
    – demure
    Commented May 25, 2013 at 3:28

3 Answers 3


I assume that at least one of the updates required a restart.

As Disk Utility found problems with the file system, it's possible that:

  • some such problems existed before your friend attempted to install updates; and/or
  • some such problems arose from a forced shut down or forced restart during a problematic installation (before an automated restart), or force whilst awaiting progress following an automated restart (impatience with the spin).

No mention of force from you or your friend, so let's assume that:

  • HFS Plus file system problems were present but unknown before installation
  • those problems caused an apparently successful installation to have a problematic end result.

In the 2013-05-06 edition of Apple's article:

A Safe Boot deletes the dynamic loader shared cache at (/var/db/dyld/). A cache with issues may cause a blue screen on startup, particularly after a Software Update. Restarting normally recreates this cache.

Not only blue screens.

Alternative symptoms might include hangs of the type seen by you and your friend – the asynchronous progress indicator with the Apple logo.

Also, I do not find that a normal restart recreates that cache. To see whether the cache was recreated, at your friend's Mac in Terminal you might run the following command – be prepared for your friend to enter her administrative password at an on-screen prompt:

sudo update_dyld_shared_cache -verify

If the output indicates that a file does not exist, then there may be:

  • a problem with Apple's documentation (inconsistent with normal behaviour of the software); and/or
  • a problem with the software (abnormal behaviour).

If the command completes with no visible output, there's no problem (the file exists and is verified).


update_dyld_shared_cache(1) Mac OS X Manual Page


Starting up in safe mode also verifies (or repairs) the main OS X volume (like Macintosh HD, which is different from verifying a drive).

Starting up in safe mode and restarting is the first troubleshooting step suggested in http://support.apple.com/kb/ts1417.

The verbose startup output when starting up in safe mode corresponds to the messages shown when verifying a volume in Disk Utility:


Please read this succinct, clear Apple Support document. Asking other Mac enthusiasts, no matter how expert they may be, for this kind of basic technical information makes perhaps less sense than checking out what Apple says, don't you think?

I found this page by googling Apple Safe Boot. Any search engine would do just as well.

  • I referenced that page to actually perform the safe mode boot up, so your sarcastic answer isn't very helpful, don't you think? I was looking for more information about it.
    – Mark Bubel
    Commented May 25, 2013 at 12:03
  • @MarkBubel you didn't mention Apple's article, but I agree that the tone of the answer is not helpful. Commented May 25, 2013 at 15:56
  • This isn't the kind of answer that makes Ask Different the kind of place people want to ask questions. Yes, we expect and ask for people to do as much research as possible before asking the questions but a negative or hostile attitude is counterproductive to effective Q&A. Commented May 25, 2013 at 19:05
  • This was not meant as hostile, just as simple as I could make it. Also, I don't see the value of repeating detailed info from Apple. Suggestions on answering such questions? Is giving the link enough? Maybe I'll have to pass on these. Thx!
    – Zo219
    Commented May 26, 2013 at 2:51
  • Mark, sorry I did not get that. You were looking for someone to expand on Apple's info ...? 'K.
    – Zo219
    Commented May 26, 2013 at 2:52

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