An appealing aspect of the Mac App Store, for many users, is the ease of transferring purchased apps to new machines. Whilst re-installing apps on new and secondary computers is generally straightforward, the process can occasionally go awry with no indication of what's causing the problem.
The most vexing of these problems arises when you delete an app, only to have the app store show it as installed. Such an error on the store's part will prevent you from installing the app on your computer. More frustratingly, if you deleted the app with no intention of using it anymore, you might continue to get notifications to update the app.
Unfortunately, this root of this problem is difficult to diagnose, considering that Apple doesn't even give you an error message. In this tutorial I will show you how to troubleshoot the issue.
App Updates on Other User Accounts
The first time I ran into this problem, it was on a shared iMac. I had installed a few apps, but had then deleted my user account. Other users on the iMac received alerts stating that there were updates available for installed apps. When the App Store window opened, the list of updates would be empty.
The problem in this case has to do with the way the operating system determines what apps you have installed, and to do this, it uses Spotlight. The index that the system has created incorrectly thinks that an app is installed, and fixing this mistake is as simple as re-indexing.
Click the Apple icon in the far left of your menubar and select System Preferences > Spotlight > Privacy tab.
Add the Macintosh HD (or whatever you might have renamed your primary HD) to this list by clicking on the + and selecting the HD, then close the System Preferences window. This will tell OS X to stop indexing this drive. No files will be deleted - All we are doing is helping reset the index that the system uses to quickly identify files for searches.
Simply closing the System Preferences window should be enough, but to make sure that OS X stops indexing, I'd recommend logging out of your account and logging back in. Click the Apple logo in the top left of the menubar, and select Log Out
Navigate back to the Privacy tab of the Spotlight pane in System Preferences. Select your Macintosh HD and remove it by clicking the - sign. Close the System Preferences window. The system will begin to re-index everything on your HD.
You can verify that this is happening by looking at the magnifying glass in the top right of the menubar, which will temporarily display a dot in the center. Depending on the size of your drive, this might take some time, and as the process can be rather intensive on the processor, you might hear the fans spin up. You can continue using your computer, but wait until the dot has gone from the magnifying glass icon before proceeding.
Open up the Mac App Store and click on the Updates tab to verify that the missing apps are no longer asking for updates on other user accounts.
Apps Incorrectly Show Up as Installed
You might run into an issue where the Mac App Store incorrectly shows that a particular app is currently installed and therefore won't allow you to download it. There are a couple of reasons why this might happen.
First, ensure the app isn't installed on an external drive or other partition. If you have another drive plugged in, the Mac App Store will recognize that you have it already and won't let you download it. If this is the case, move the apps by dragging and dropping them onto your primary drive, eject and unplug the external drive, then trash the app and empty the trash. Re-open the Mac App Store and navigate to the app's page.
If you don't have the app lurking elsewhere, the root of the problem is a bit deeper. To fix the problem, we are going to reset some caches and property lists (also known as "plists"). To do so, we will need to trash a few folders and let OS X rebuild them.
We can either navigate to the folders, or to save time, we can use a few simple Terminal commands.
Open Applications > Utilities > Terminal.
We are first going to reset three plists. Copy the first line, paste it into your Terminal window, and hit enter. Repeat for the remaining lines, (again, separately).
Next we are going to reset two caches. Again, copy and paste each line separately into Terminal, hitting enter after each.
rm -r ~/Library/Caches/com.apple.appstore
rm -r ~/Library/Caches/com.apple.storeagent
Follow the steps above regarding resetting the Spotlight index.
Restart your Mac. Navigate to the Mac App Store and download the the desired app.
In this Quick Tip, we have run through a few easy ways to troubleshoot a common problem with the Mac App Store. While you may experience these problems separately, both are rooted in errors in how OS X determines what apps are installed. In order to fix this, we simply use Terminal to reset the files that the operating system uses to make that determination. If this was your first time using Terminal and are interested in learning more about it, I highly recommend checking out James Cull's excellent tutorial on Terminal commands for beginners.
The troubleshooting steps outlined in this tutorial should help resolve the most common problems with Mac App Store download errors. Have you run into another problem with the Mac App Store's download process? What steps have you taken to fix it?