Hi there! :-)
I know that I can hide my files on my N9 by putting a dot before the name, but I still see these folders on my PC.
How can I hide them permanently from the N9 and the PC at the same time?
any help will be appreciated... :-)

cheers.

Move it to /home/user/ done
or remove it, it gonna be completely gone,

or rename it...
photo.jpg --> visible
.photo.jpg --> hidden

Do take note that storing in /home/user/ will take up space from Application data (userfs) partition so make sure you don't end up putting too many files/folders in there...

Like this

Firstly, I have no porn
Then, LOL!

• firstly, *nix based systems are... file based
  everything is in a file, somehow, somewhere; thus if someone gets access to the file system, they will eventually find it
  thus, keep your N9 safe, and data on it will be as safe
• 2ndly, hide stuff in plain sight.
  depending on what you want to hide you could consider encrypting it into a picture (aka Steganography). you probably have plenty of pics on your N9, thus...
  or put it in an encrypted ZIP (or TAR.GZ or... the more exotic, the better of course) with the -e or --encrypt option on the ZIP cmd line and put it e.g. in an software directory where you have tar.gz source packages (no need to do anything with them, obviously, just download a few); rename the file from file.zip to file.tar.gz et voilà...

if people are determined enough to find whatever you are hiding, they will find it, no matter how well you hide it. if nobody should see the information, destroy it.

and don't discount chance, e.g. someone seeing the picture and thinking «o, this looks nice, maybe i could zoom / crop it and use it as a background...» or «o what is this package doing? maybe i can try and build it and check it out....»

This is good - but where / how is the .photo (hidden) viewable?

You click on it using a file manager...It'll open with the built-in gallery app but once cosed won't be viewable in gallery...

fankoo