Mac Geekery

Answer

You need to create an entry in your filesystem table to tell the OS not to mount that specific volume. This is relatively easy.

First, we need to uniquely-identify the volume to hide. The name’s not good enough (how many “Untitled” CDs have you burned?), we need something more unique. Luckily, every HFS volume has a UUID identifier that we can use for this. This way, renaming the volume will not change this behavior.

Using diskutil we can see what the UUID of a volume is. With the disk connected run this:

diskutil info /Volumes/[name]

Look for the UUID in the output and copy it to the clipboard. Now that we have a UUID for the volume we should create ye olde fstab file. Save something like the following in a new file called /etc/fstab:

# Identifier, mount point, fs type, options, dump order, check order
UUID=73853D0B-489C-1C22-0148-62BE288C102F none hfs rw,noauto 0 0

Now reboot (for internal drives) or unmount, disconnect, and reconnect the drive (for external drives) and it should not mount the partition we identified. If things don’t work, start at the top and try again. If things still don’t work, leave a very, very detailed comment below.

And I bet you thought it was a flag somewhere. Well, it used to be in Mac OS 9, but no longer. In Mac OS X, diskarbitrationd is the king. Now, this just changes the automatic behavior. You can still manually mount this disk in Disk Utility or via the command line. If you don’t want the partition available at all, you’ll need to move the superblock elsewhere and manually identify it on mount. That’s a tip for another day.