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You can download your iTunes Store purchases free of charge again at any time. Any content that did not come from the iTunes store would have to be re imported from their original sources.
iTunes 11 for Mac: Download previous purchases from the iTunes Store
You can download many types of content previously purchased from the iTunes Store to your devices and authorized computers. This way, you can add items to your devices and computers without having to sync or copy files. It’s also useful if your iTunes files are lost or deleted for any reason.
Previous purchases may be unavailable if they are no longer on the iTunes Store.Note: Some types of previously purchased content might not be available in some countries.
Download previous iTunes Store purchases to an authorized computer
- Click iTunes Store near the top of the iTunes window.
- Click the Purchased link at the right of the iTunes Store homepage (below Quick Links).
- In the page that appears, select a category.iTunes shows you which of your purchases are available for download. You can view all of your past purchases, or just the ones not currently in your iTunes library. Items already in your library aren’t available for download.If you select Music, you can view your purchases by song, artist, or album. If you’re viewing by album, click an album to see which songs on it are available for downloading.To search for an item, type its name or a keyword in the search field, and then press the Return key to view your search results.
- To download an item, click its Download button .To download all available items, click Download All.
There is relatively new service called iTunes Match which stores your non iTunes Store purchases in the Cloud so they can be downloaded to your authorized devices straight from the Cloud.
All the best,