iTunes, how do i save my music library to external storage?

See Change where iTunes files are stored on PC - Apple Support

It is perhaps worth noting the officially described method to change your media folder to a new path often doesn't work, with the path resetting to the previous location (iTunes Media inside your iTunes folder) on each subsequent launch of iTunes. A better approach is to move the entire iTunes folder from the current location to the root of your prefered drive. E.g. Go to C:\Users\<User>\Music and select the iTunes folder, press Ctrl+X to cut from the current location, then browse to the drive you want to use instead, e.g. D:\ and press Ctrl+C to complete the move. Once files have finished transferring press and hold Shift as you launch iTunes, click the Choose Library button, then browse to and open the library file at D:\iTunes\iTunes Library.itl, or whatever drive letter you used.

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