There are several hints on this site for unmounting a partition at login, but a better way to make this happen can be used. You need to create an entry point in your filesystem table to tell the OS not to mount that specific volume, and this is relatively easy. In Mac OS X, and under Linux and others unix systems, the volume mounting configuration file is located in /etc/fstab. This file in Mac OS X Tiger is empty in Panther, its commented, but type man fstab in Terminal to read some important information.
via Prevent a given partition from mounting at boot – Mac OS X Hints.