https://apple.stackexchange.com/questions/145683/permanently-disable-ignore-secondary-disk

First get the UUID of the partition you want to prevent from mounting... You can get this in Disk Utility "Get Info".

Now open your /etc/fstab file for editing:

sudo pico /etc/fstab

In this file add the following line (use your own UUID naturally):

UUID=[your UUID] none hfs rw,noauto 0 0

And save the file.

Restart for the change to take affect.

You can always mount it manually in Disk Utility.