[Q] Is factory data reset safe on ics 4.0.4?

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http://forum.xda-developers.com – Hello guys just got myself a pre-owned i9100 s2 running on ics 4.0.4, i wanted to know how much internal storage is the phone meant to have? I connected to my laptop and in explorer it tells me i have 6.87gb free of 11.4gb. I am a little worried as i was expecting it to have more internal storage than that (10gb +), i currently don't have any thing important on the phone. I was wondering is it safe to factory data reset the phone via Settings>Backup and rest>factory data reset? and also do i need to tick the "format usb storage" option? I am hoping to get as much storag (HowTos)