initialize hard disk mac pro

Hi everyone, before sending to apple my macbook pro for a battery replacement, I want to delete all files in it. ( I already have time machine backup on external hard disk )

So I open the laptop in recovery mode, utility disk, and clicked " initialize" macHD.

The question is: I saw 2 different macHD in recovery mode , one smaller ( 20gb) and one bigger (100gb). I initialized the smaller one, and then the screen turned all black showing a "?" .

I want to know if I deleted all files in it or only the OS ( but all data are still in the HD).

Any opinion about this? is there a way to know if mac is empty or not?!

thanks in advance

Andy