At the time, we didn’t bother to figure out which tracks were missing, but for my wife’s birthday this year, I decided I’d finally find those tracks and bring them over to her Mac. I’d assumed it would be a painstaking process: I’d need to look for a couple hundred songs—out of thousands—that existed on the old PC but not her Mac. And, of course, her library has grown substantially since the switch, so comparing the two libraries would be far from simple.

Luckily, I discovered , a free Mac app that makes quick work of the process. You just feed it a pair of iTunes-library XML files, and it shows you which songs exist in one library but not the other.

In my case, I went to iTunes on the Windows PC and chose File -> Library -> Export Library; I copied the resulting library file from the PC to my Mac. Then I used the same library-export command on my wife’s MacBook. I now had two XML files, each containing complete information about the contents of that computer’s iTunes library. Of course, Mashduo would also work with two library files from Windows PCs or two from Macs.

To use Mashduo, you launch it and drag one library file (in my case, the MacBook’s XML file) into the space on the left of Mashduo’s window, and the other (in my case, the Windows PC’s XML file) to the space on the right. Those spaces are labelled Your Name and Friend’s Name, because Mashduo is pitched as a way to compare your library with a friend’s. I’m sure it’s a fine tool for that, but that wasn’t my goal.

Tapping the Compare button displays a Venn diagram listing the number of tracks unique to each library, along with the number of tracks the two libraries have in common. For my test, the utility took less than a minute to compare nearly 7000 tracks. When the process completed, I could see that my wife’s PC had 322 tracks that weren’t on her Mac.