sadhuram, many thanks. I will appreciate your feedback on these three things:
1. FIRST
After
sudo nano /etc/fstab
I pressed the carriage return and I got something like this (without the lines ---):
--------------------------------------------------
UUID=3C236DAC-45CD-8ADB-8J8A-KI87K756L875 none hfs rw,noauto
UUID=9DKL76JH-859G-9865-87L7-TN59694I6K47 none hfs rw,noauto
--------------------------------------------------
and then I pasted below what you indicated to read (without the lines ---):
--------------------------------------------------
UUID=3C236DAC-45CD-8ADB-8J8A-KI87K756L875 none hfs rw,noauto
UUID=9DKL76JH-859G-9865-87L7-TN59694I6K47 none hfs rw,noauto
# fstab
#
# Identifier mount point fs type options1
#
UUID=U4766497-8TBE-9Y79-M4IH-K7OT4G49475H none hfs rw,noauto
--------------------------------------------------
I understand that it is not required to press the carriage return after pasting that.
If now I press/type:
ctrl O
nothing happens.
If then I press/type:
ctrl X
nothing happens either.
2. SECOND
Imagine that the booting OS X 10.8.3 above is an external disk used to boot two different iMacs (one at work and other at home, for instance). One iMac has the UUID indicated above, but the other has a different UUID, of course. How to prevent that none of such internal Mac hard disks automounts when either of the iMacs (at work or at home) is booted from the external disk?
3. THIRD
On the other hand, how to reverse the unmount feature and make such iMac internal disks automatically mount (if required in the future)? In other words, how to revert this change if required in the future?
Thanks again.