Originally Posted by XS_31
Usually, a factory reset erases /data without erasing /data/media.
So you don't lose videos, musiques and all personal files you put on internal storage (which is mounted as a SD card by the system).
It's just with stock recovery a factory reset completely wipes the internal sdcard, that's the whole point of a factory reset.
Anyway I found the option to format the sdcard in the menu via CMW recovery, just seems an odd way to do it.
Maybe it's done that way to protect the user in-case they do a reset and lose the zip file their trying the flash, that would be pretty funny.