Your iTunes library is very important, housing music, TV shows and more. However, with all this important media to look after it can swell to huge proportions. There may come a time when your iTunes library outgrows your Mac and there's no space left - especially if you recently transferred from a desktop with a huge hard drive to aMacBook Air, say, with a relatively tiny SSD.
Should this happen, you've got three options: madly prune your media collection; move the iTunes library to a different Mac with more disk space; or copy the iTunes library to an external drive. We figure you can take care of the first option yourself, so we'll concentrate on the other two.
How to move your iTunes Library to another location: Transfer your iTunes library to another Mac
When migrating to a brand-new Mac with a similar amount of storage space to the old one, OS X's Migration Assistant is your best bet, automatically transferring content for you. However, it's also possible to manually transfer your library to another machine, eventually leaving the one you copy it from with lots of free drive space.
On the Mac you want to copy from (the source Mac), go to System Preferences and open Sharing. In the services list, turn on File Sharing. On the Mac you're copying to (the destination Mac), your source Mac should appear in the Shared section of theFinder sidebar. Select it and click Connect As. (If it doesn't appear, use Go > Connect to Server from the Finder menu, and type in the server address that's shown in the aforementioned Sharing pane.) In the sign-in dialog, select Registered User and type the name and password for the source Mac.
Navigate to ~/Music on the dialog showing the source Mac's file system and drag the entire iTunes folder to the equivalent location on the destination Mac. Depending on the size of the library, the transfer could take a long time.
(Note that if you've got a back-up clone of the source Mac, connecting that to the destination Mac and copying from it instead might be quicker. Also, be aware that this kind of copy is destructive, and it will overwrite all iTunes content on the destination Mac. If there's already unique content in iTunes on that Mac, copy it elsewhere, if necessary.)
Once the copy is complete, authorise the destination Mac to play content bought from the iTunes Store that has digital rights management. Test a number of media files to check that the copy worked fine. (For extra safety, it's a good idea to make a temporary second copy of it to a separate drive, just in case, though.) If you're happy, you can now delete the iTunes library from the source Mac. Remember to deauthorise any old Macs you're planning to no longer use.
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