How to prevent automount of a partition in El Capitan
Tetsujin and user3439894's comments and observations prompted some testing.

Turns out, there is more than one UUID when you look at drives and partitions and the 'diskutil list' command doesn't report the UUID that needs to be used in the vifs/fstab commands. 

  root %&gt; diskutil info disk1 | grep -e UUID


Shows that there are Volume, Disk / Partition, LV,LVF and LVG UUIDs .. 
I'm only interested in the first two!

When I enter diskutil list I get the "Disk / Partition UUID", when I enter diskutil info disk1 I get both the "Disk / Partition UUID" and the "Volume UUID" (and more ..) I also get the "Volume Name" (the disk label) 

Some tests I performed with the Volume - UUID and the disk label indicate: 


the UUID in vifs/fstab is the "Volume UUID", don't use tabs! 
the UUID command in vifs/fstab doesn't work with the "Disk / Partition UUID", tab/space makes no difference here
when using the "LABEL" syntax, don't use tabs!


Here are the two examples I got to work:

UUID=&lt;Volume UUID&gt;&lt;SPACE&gt;none&lt;space&gt;rw,noauto

LABEL=&lt;Volume Name&gt;&lt;SPACE&gt;none&lt;space&gt;rw,noauto


You can find the &lt;Volume UUID&gt; and the &lt;Volume Name&gt; of your internal disk by running

diskutil info disk1 | grep -e "Volume\ Name" -e "Volume\ UUID"


On my system the external disk-info shows up for disk2s1 and disk3s1