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You don't have to use Linux to store files in the Local Storage directory on your SD card,
It is less common to keep music there, because most of us have SD cards that are much smaller than yours. So keeping music on a flash drive or on the network is more common, especially for people with a large music library, and an 8~16 GB SD card.
To store files on the SD card, you first have to make sure the SD card partition where the Local Storage directory is located is big enough for what you need. If you have not done it already, you have to expand the partition using the directions in the DOCS section here.
http://www.runeaudio.com/documentation/(Look for the section on how to extend a partition.)
Unfortunately, to expand the partition on the SD card, you do have to use a few Linux commands, which are listed in the docs, but in general, using Linux is not a requirement, and after the partition is expanded, you can copy files into it over the network.
