How can I hide a partition (or prevent it from mounting)?
Partitions that are mounted in Mac OSX are generally present in the file /etc/fstab that is read upon boot. If you just comment out (i.e. put a # in front of it, and save the file) the line that referes to the drive you do not want to mount, it will not mount upon next boot.

edit
FYI. I am not sure which OSX you are on, but since OSX Lion the implementation of /etc/fstab has changed:

https://discussions.apple.com/thread/3582968?start=0&amp;tstart=0