I wanted to manage music on an SD card, as that's the way a Nintendo DS gets its music. I wanted my son to have his music on his DS so he didn't have to keep track of and charge an MP3 player and a DS.

First, Rhythmbox on Ubuntu 10.04 wouldn't recognize the SD card. Fortunately, I'd stumbled across the solution to that a few weeks ago: put a file called ".is_audio_player" in the root of the SD card.

However, it still wouldn't copy m4a files over. The DS only plays m4a files, not mp3. I discovered that you can actually put content in the .is_audio_player file to control the behaviour of Rhythmbox. It was still hard to figure out what to put in, but finally I looked up what Rhythmbox uses to manage an iPod, and put this in the .is_audio_player file:

folder_depth=99
output_formats=audio/mpeg,audio/aac,audio/x-wav,audio/x-aiff

Worked a charm after that.

P.S. My son's doing some serious scratch with the Police on his DS right now.