Windows: You may already know that when you buy music from iTunes, Amazon, and other digital music stores, some personal information like your account ID or username are embedded in the track metadata. Desiccate is a free Windows utility that scrubs them clean.
Desiccate is simple to use: as long as you have .NET Framework 2.0 or higher installed, you can launch it directly from the link below and run it against your files (it's deployed as a server-side app hosted by the developer). When you do run it, you're presented with three checkboxes that represent different types of data you can remove.
PRIV and COMM data are often used by Amazon and COMM and UFID are used by iTunes, but keep in mind that COMM will remove any comments from your music file's tags—including those you may have added yourself. You can even tell the app to take a test run against your files to log what it would remove before you commit to the changes. As always, make sure you have backups of your files before using an app like this: the dev notes that if your music files are corrupted, running any tool like this may corrupt them further. When I ran it however, it cleaned my music files with no issues at all.
It's worth noting that the personal info embedded in each track is how music retailers combat piracy: if the file appears on a file sharing network, it can be traced back to the person who uploaded it. Unfortunately, it could also put you in a bad position if your computer is lost or stolen, or if your computer is compromised and someone takes the opportunity to snag all of your music. Use your judgement, and it should go without saying: don't do anything illegal and/or stupid.
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Desiccate Scrubs Personal Data from iTunes and Amazon-Bought Tunes