I have an iRiver 4GB portable mp3 player, but I use it to play Vorbis ogg files.
I have a lot of tunes and every once and a while I like refreshing the whole bunch.

I wanted something simple that I could just point to my Music directory, then point to my USB connected iRiver and walk away.

I wrote my own little Java app to do it. (License: GPL)
Source is attached. compile it and add it to your class path and you are good to go.

Here is the commandline syntax:
tgutwin@warp4:~$ java  ca.bc.webarts.tools.RandomFileSelector

SYNTAX: java ca.bc.webarts.tools.RandomFileSelector [-clear | -testing] [-max] [-type fileExtension] musicDirPath transferToDirPath
  You *must* supply the directories on the commandline, other options are optional.
  
   Filetype defaults to ogg

  An Optional argument can be specified:
   '-testing' to allow no actual writes
   '-clear' it will contents of the device dir first
    '-fill' will run until the device is full ELSE it loads 250 songs at a time

 EXAMPLE:
    java ca.bc.webarts.tools.RandomFileSelector -clear /var/media/snd/ogg /mnt/usb-storage/music
 or
   java ca.bc.webarts.tools.RandomFileSelector -clear -fill -type mp3 /var/media/snd/ogg /mnt/usb-storage/music



Try it out.