Howto Stop Automount at boot?

Sure.

Use Lingon to create a "USERS DAEMON" to run a script containing this at startup:

/usr/sbin/diskutil unmount 'EB408EBE-27F1-3A32-A628-C6575F2C422E'

That string in single quotes is just an example. You need to get the actual volume UUID for your volume using Disk Utility's "Info" option on the volume (not the disk). I prefer to use the UUID because it never changes unless you delete the volume (partition).

You can remount the volume using this command:

/usr/sbin/diskutil mount 'EB408EBE-27F1-3A32-A628-C6575F2C422E'

I actually use this command in my situation:

/usr/sbin/diskutil eject 'EB408EBE-27F1-3A32-A628-C6575F2C422E'

I do so because I want the whole disk ejected and it seems to spin the drive down until it is mounted again.

This disk that is ejected at startup is used by SuperDuper!. I have SuperDuper! setup to backup the boot volume to the this disk. SuperDuper! automatically mounts the volume and does a smart copy. When that is done, I have SuperDuper! run the script to unmount the disk again. So the disk is mounted only during the backup process.

Scott