To work with the FUSE in Rhythmbox, you have two choices: MTP mode or MSC mode. The MTP mode is the Media Transfer Protocol which the Windows Media Player uses. There's a plugin for Rhythmbox which lets the player show up and you are able to manage the player from there.
The MSC mode is Mass Storage Class mode which brings up the player like any other USB stick or external harddrive. Rhythmbox won't recognize it as a player in this mode but you can force it by creating a file
.is_audio_player in the root of your Sansa.
Make sure to put following contents into that file:
audio_folders=MUSIC/,PODCASTS/,AUDIOBOOKS/ output_formats=application/ogg,audio/mpeg,audio/flac,audio/x-ms-wma,audio/aac,audio/mp4,audio/audible input_formats=application/ogg,audio/mpeg,audio/flac,audio/x-ms-wma,audio/aac,audio/mp4,audio/audible folder_depth=1 playlist_path=PLAYLISTS/%File playlist_format=audio/x-iriver-pla
audio_folders tells Rhythmbox where to put music files. Without that line all songs will be put under the root and thus the player won't recognize them.
output_formats describes all formats the player can create, i.e. by the internal recording feature.
input_formats describes all formats the player recognizes to that Rhythmbox can decide whether to transcode a file or not.
folder_depth influences the directory structure Rhythmbox creates. A
folder_depth of 2 creates a
Artist/Album/Song.mp3 structure. A 1 creates a
Artist - Album/Song.mp3 (which is what I prefer) and, I guess, a 0 doesn't create any folder but just puts the files into the
playlist_format describe the location for playlists and what format the player supports. (I'm not sure whether the FUZE supports
/usr/share/hal/fdi/information/10freedesktop/10-usb-music-players.fdi lists the options usually set for FUZE players. Just search for the string “Fuze”. But I guess the PnP-ID of the newer models isn't in that XML file.