I wrote this program because synchronizing mp3 files from my music collection to my Sansa Fuze mp3 player with sd slot (seen as 2 different storage units from the PC) was a pain to deal with.
This tool will synchronize your music collection between your PC hard drive and your player internal memory + SD card.
Kinda like a crude Robocopy from a source to 2 destinations.
Principle:
A hash is computed for each file on both PC and player:

• Each file that's present on the player but not on the PC will be moved from the player to a chosen directory on the PC.
• Each file on the PC source that's not on the player will be copied.

Known limitations:

• 10Mo of available space will be kept on the player, and 10Mo on the SD card (overstuffed devices seldom works well).
• First hash compute is really long, but wait, it will be cached for future use, so the second run will be way quicker.
• Files will be copied within their parent directory, but no further: A file c:\foo\bar\baz\quux.mp3 on the PC will be copied as baz\quux.mp3 on the player.
• A modified mp3 tag on the source will be considered as a different file (hash based comparison) => hence the file will be copied all over again.
• For 2 discs albums, a duplicated folder.jpg file will only be copied once.
• Only works in MSC USB mode (I guess this tool is pretty useless in MTP mode, isn't it?).
• Only works in console mode (but a GUI will be easy to shape and to interface with the core dll).

Command line:

Mp3Sync.exe -s SourcePath -d1 DestinationPath1 -d2 DestinationPath2 [-dbin DestinationBin] [-ext ExtensionString]

• SourcePath: Path of the source to synchronize with, like "c:\music"
• DestinationPath1: Path of the destination 1 (player music dir), like "H:\Music"
• DestinationPath2 (optional): Path of the destination 2 (player sd card music dir), like "I:\Music"
• DestinationBin (optional): Were to put files found on the player, but not in the source, like "c:\MusicDeletedFromPlayer" - If not provided, extra or out of date files will be deleted.
• ExtensionString (optional): Pipe separated extensions of the files to synchronize - If not provided, "".mp3|.jpg"" will be used.

Note: This tool might be used as a generic synchroniser, but there are tools really more suited than this one if it's a one to one synchronization.
This program is distributed under the GNU General Public License.