Mac App Store update stuck (Installing - Calculating...) I opened GarageBand for the first time, to find that most of the loops required me to download the Apple Loops from Software Update. I agreed to this.

The App Store opened and gave me the option to install the update from Updates - I clicked Install and waited for the download to complete. It did, and then the installation began.

About half an hour later, the update is stuck on (Installing - Calculating...) This doesn't seem normal - the last time I installed the Loops, they were done in about ten minutes.

Force quitting and re-opening the App Store doesn't help, nor does a reboot. When I open the App Store, the update is still at the same process.

Is there anything that I could do to get the update to finish installing?

Here is a screenshot of the App Store in this state:
In an act of desperation, I managed to fix the issue by:


Deleting /Library/Updates
Doing 

sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate.plist RecommendedUpdates
sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate.plist DidRegisterLocalUpdates


in a Terminal session
Restarting the Mac


The update is re-downloading now.

Interestingly, before I tested these steps, I found the update ready to install in /Library/Updates, however when I tried to install the PKG file, Installer claimed that another installation needed to finish (?). Deleting it and restarting fixed the problem.