Itunes Library to new Imac?

If you gather the whole library into a portable shape on the external drive then you can connect to and use it on that external drive with a different computer. When moving to Music from iTunes your old iTunes library needs to be converted to the new Music format. You can choose to save the converted library on the external drive along with the media. The converted library will use the same media folder as before by default. You can do something similar with the TV app. You don't get any control over where books and podcasts are stored. These and the artwork caches for the various libraries will be created on the system drive.

Changing the media folder doesn't change which library is opened, or cause Music to scan the folder for content. You need to press and hold down option as you launch Music, keep holding until asked to choose or create a library, click choose, then browse to and open the old iTunes Library.itl file. You then get to choose where to save the converted library. I would suggest <External>/Music so that Music stops with an error if you try to start it when the drive isn't connected. The prevents the media folder being unexpectedly reset.

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See Move your iTunes library to a new computer - Apple Community.

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