How do I format internal SD?

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http://forum.xda-developers.com – Hi I have a huge problem. Sms/mms app won't run. It crashes after boot and any time I try to start it. I have made several clean installations of stock ROM with no result. Including wiping everything of course. But I can't figure out how to format the internal SD. There's files left there no matter what I do. How do I make a true format so I can start over from scratch? Sent from my Goner Simpsung S3 - XDA dev app. (HowTos)