You will have to reinstall OS X. If 10.8.4 is on the App Store site then that is the version you should get. Otherwise, it will be 10.8.3.
Reinstalling Lion/Mountain Lion Without Erasing the Drive
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu.
Reinstall Lion/Mountain Lion: Select Reinstall Lion/Mountain Lion and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
If you end up with 10.8.3, then you can update by downloading and installing 10.8.4 Mountain Lion Update v10.8.4 (Combo).
Reinstalling OS X from Recovery Mode now (obviously writing these lines from another machine, an older iMac which incidentally updated without a hitch) ... took several tries until it actually rebooted and began installing again.
In the meantime: what's the cause of this? Seems I'm not alone with this issue ... Any ideas?
Will report back if/when I've managed to kick my iMac back into submission.
The most common cause is some type of dysfunction in the underlying system being updated. In most cases simple repairing the hard drive and permissions first will set things straight. If the dysfunctional system is in really bad shape then reinstalling OS X may be the only viable option short of erasing the drive and starting from scratch.
In some cases the problem is that the system being updated is best done using the Combo updater rather than the incremental updater because this can fix any extant problems arising from previous updates. As a matter of practice I always repair my hard drive and permissions before updating, and I always use the Combo updater. It must be a good practice because I've never had an updating problem.
Just for the sake of completeness, I hd a similar problem with a MacBook Air 2011. After the update the system just hung with the grey Apple Logo and the little spinning wheel below.
After consulting this and various other web pages, I began by powering off, then starting in Safe Mode (Hold down Shift Key while starting Mac). This got me to the normal login screen and after logging in the Mac showed it was running 10.8.4, and a normal shutdown and restart has got me back to normal functioning.
Hopefully this might help any Macbook Air users out there as I couldn't find any specific comments on the web on this problem with Macbook Air.
Before you re-install... you might not need to.
Try booting into safe mode via Shift-Command-V during start up (instead of just Shift).
I just had this exact problem on my nearly new MBP Retina 15". Was on 10.8.3, updated via app store to 10.8.4. Updates downloaded and installed, then just before automatically restarting to finish the updates, I got a dialog saying not all could be installed successfully with the only option to restart (as far as I can remember... this was late last night). Machine shutdown automatically and tried to boot itself, and just hung on white/grey screen with apple logo and spinning progress wheel underneath. Left it on this screen overnight. This morning, it's still the same.
Couldn't boot into safe mode using just shift key - it just went into same grey screen. Tried repair disk and repair permissions in disk utility - still wouldn't boot into safe mode or otherwise.
Then, I read this http://support.apple.com/kb/TS2570 - hold Shift-Command-V during start up (instead of just Shift). After a load of command line stuff being spat out at startup, it started straight back into where I Ieft off before this whole disaster. About this computer now reports that I'm on 10.8.4.
After this, I restarted normally, and everything appears to be fine.
Shoddy update, Apple. I can honestly say in 15-20 years of using Macs and Windows, I've never had a 2 month old machine not startup after a system update...
I have the same problem on my imac after this update only after the usual grey screen and apple logo I get a chevron - like pattern across the screen. My macbook pro is asking me to update to 10.8.4 and I am now loath to do it as it is the only internet access I have other than ipad etc.
Tried shift/command/v but same happened after the command stuff.
I can however boot in safe mode!. i cannot lose the data that I have.
Why is this all happening - its why I switched from Microsoft!!!
You could try booting into recovery (CMD + R) then repairing permissions, if you haven't already? Then try the shift + cmd + v maybe. That's what worked for me, but I didn't get the chevrons you mention.
Agree this is not expected on a Mac. Windows updates have never given me any trouble!
My problem is similar, although I seem to get farther than many of you. My iMac, purchased this year, boots up and displays the icons for the two user profiles on my system. However, when I click on either of the icons I get the spinning rainbow beach ball. I have already booted and used Disk Utility to verify the disk, repair the disk and repair the disk permissions. All of those operations indicated that there was no problem.
When I booted using Safe mode (holding shift), I wasn't able to use my trackpad. When I booted using Shift+Cmd+V, my trackpad worked and I was able to click on a profile icon and get a password entry field... and then the spinning beach ball returned. The second time I booted this way I was able to quickly enter in my password... but then the spinning beach ball returned yet again. The third time I booted this way I never got the password prompt.
I guess my next attempt will be to try the reinstallation of OS X from the Restore partition.
Unfortunately, because I've gotten so used to NOT having any problems with Apple updates, I was complacent and did not make a Time Machine backup prior to doing the upgrade. :-(
Just to update... the reinstallation of OSX from the Restore partition did work, although I had a scary moment when it rebooted and then just froze at a white screen with the Apple logo. I rebooted again and got to a grey screen with the Apple logo (but where I could use my trackpad).
I then rebooted with Shift+Cmd+V and thankfully got to the login screen where I am able to login.
My system seems to be all intact and is currently at OS X 10.8.3.
First thing I'm doing now is to do a Time Machine backup because I've unfortunately learned I can't be so complacent and trust Apple OS X updates.
And... I think I'll skip the 10.8.4 update for right now. Perhaps a 10.8.5 will come out that will have a higher quality level. :-(