So, for the past couple weeks I have been baffled - every time I try to update apps on any of my Macs, the update fails and I am presented with a dialog box that says "To update this application, please sign in with the account you used to purchase it".

One big problem - I already was.  I only have one iTunes account.

So.. yesterday, I decided to monitor the console - in doing so, I noticed that each time I tried to update, I saw an error with mds - a spotlight derivative.

Interestingly enough, all of my mac machines use SSDs.  Because they use SSDs, I have made several enhancements to my machine to prevent excess drive writes (to prevent un-necessary wear on the SSD).  One of the common "Enhancements" I make is to completely disable spotlight, and also add my main HDD to the list of locations NOT to index.

So, I wondered if this could be causing the issue.  I re-enabled Spotlight, and removed my drive from the blocked locations.  I waited for a while to allow spotlight to finish indexing (the magnifying glass icon in the upper right corner of the finder will have a "dot" in it while it's indexing".  Once it finished, I reopened the App Store and guess what - everything worked fine.

Apparently, the app update process requires spotlight.  Maybe this is done to "find" apps in the event that the user has moved it from it's original location?  Not sure, but - for now I'll leave it enabled I guess.  

If you're having trouble, too - follow these steps and things should start working again!