You can create a text /etc/fstab, as it does not originally exist. In the Terminal, there is a man page for fstab(5) that gives the layout.
UUID=x none hfs rw,noauto
The 'x' is the UUID number string of the drive you do not want to mount on boot. You can get this from the Disk Utility Info button, which calls it the File System UUID, or within the Terminal as:
diskutility info /dev/disk0s2 (or whatever internal device string)
where it is referred to as the Volume UUID. All whitespace in the /etc/fstab file is a single space.
Here are the permissions, so you will need sudo privileges to create this file:
Before you reboot, make absolutely certain that your external boot drive is established in the System Preferences : Startup Disk panel.
Once rebooted, your internal drive will no longer appear on the Desktop, and it will be grayed out in Disk Utility. You can however, click on the faded drive name in Disk Utility, and then select the Mount button should you need to access it. When done, you simply click the Unmount button.
The above is what I did with the internal 500GB rotational drive in my 2011 Mac mini, after I installed a second, internal drive (SSD) as the boot device.
