I did the strace,

oldumount("/drive0");
fork(); // runs some program that returns "/dev/hda4: clean, 5547/" probably fsck
mount("/dev/hda4","/drive0","ext2",MS_RDONLY|0xc0ed0000,0x210f65c)
// does some more stuff
mount("/dev/hda4","/drive0","ext2",MS_REMOUNT|0xc0ed0000,0x210f65c)
// does a lot more stuff (probably the actual sync)
mount("/dev/hda4","/drive0","ext2",MS_RDONLY|MS_REMOUNT|0xc0ed0000,0x210f65c)

So it looks like we're being bitten by the fact that we can't perform a filesystem check on a mounted filesystem. The actual sync should be just fine.

At the end of the sync, the player kills itself and is restarted. Hmm, would it be possible to kill any process that has an open file reference to the filesystem at the time of umount. Then we could use ;@EXEC /drive0/bin/telnetd and it would just get killed when the sync is started, but will restart whenever the player comes back.