I have similar problems. I got my 5.5 ipod to mount on my mac osx and started to copy files to it. After I had been transferring files in aprox 10 min. it stopped suddenly and the disk unmounted. I havn't been able to mount it again in OSX.
But it mounts fine in windows when I start my virtual machine (I'm running windows 7 in Parallel (virtual pc) on my mac).
So I managed to copy the rest of the files via the virtual windows, but it was quite unstable, because as soon as I returned to the mac-interface, windows would stop copying and I had to start over again.
all the best
jbrandtp