How can I move MySQL data directory on Mac OS 10.5? (and related questions)

Question

I've managed to mess up my MySQL database (on Mac OS X 10.5) and need help recovering!

I tried to add an index to a fairly large table (190 million records) and in the course of this, I ran out of disk space. Subsequently realized that the partition with the data directory is too small and so I need to move it.

Initially I thought that I would just copy the data directory to another location, then bung a symlink in place of the original data directory.

BUT it refuses to move!

sudo cp -r /usr/local/mysql/data .
cp: ./data: Permission denied

(I have stopped the mysqld process before attempting this move)

Help!