i just discovered another reason for this error occurring: Hijack. Or rather, Hijack still having files open because someone is streaming or downloading an MP3 off of the empeg. Normally, when the player application is closed all MP3 files are closed too, but since the Hijack HTTP server resides in the kernel and is still running while a sync is going on, it is possible for MP3 files to still be open and prevent the disk from being unmounted...

/Pepijn