Save file Samsung S2

Maragost

Hi to all
I've alredy done this question some week ago but i've got busy at work and so i'm going to format my phone just now.
What i would like to know is how can i keep my save?
I asked help to a friend which is more thane me into these things and he said that i must root my phone because otherwise even i find where it is supposed to be, i won't see the file. Is this true?
I would like to don't have my phone rooted since other friends told me that it may cause problems with certain programs and once the phone is rooted i'll be needed technical help more than before.

So in the end, what do i have to do to keep my save file? I don't have an external SD at the moment. If i get one and i try to transfer the whole game, will it transfer the save file too?