Umm. Maybe there's a process running that's using /drive0.. Quite possibly emphatic.

Run this:

ps -ef

And if you see anything running on the music partition (emphatic, or whatever else) kill it by noting the process ID (the number in the first column) and typing:

kill 123

assuming the process ID is 123. Then try running rom and ro again to mount read-only. My guess is you botched the chmod command and accidentally ran emphatic.