forums.androidcentral.com/asus-fonepad/302368-will-factory-r...

This thread has an authorative answer on what happens when you do a factory reset.

forum.xda-developers.com/galaxy-nexus/help/wipe-internal-sto...

This thread tells of how a Factory reset only wipes /data in a CUSTOM recovery. In stock recovery it handles the storage partition also.

forum.xda-developers.com/galaxy-s2/help/factory-reset-format...

A forum thread discussing this topic giving some assurance as to what happens

forum.xda-developers.com/galaxy-s2/help/safely-formatting-in...

This is an excellent question and answer for formatting internal storage on Android.

After some 2+ years of constantly changing ROMs, I've noticed that my i9100 shows more often than not the message of "your storage is getting full" or ...

www.wikihow.com/Erase-Internal-Storage-on-Android

If you need to sell your Android, or just completely reset it, you can do a factory reset to erase the internal storage.

According to helpfulness feedback, this only deletes user data, not internal storage.

android.stackexchange.com/questions/56429/how-to-properly-fa...

This forum thread discusses what happens when you do a Factory Reset and that it does not touch the exteranl storage.

forums.androidcentral.com/t-mobile-galaxy-note-2/333410-what...

I have searched but couldn't find any info. what exactly happened when we do 'factory reset'? What gets restored?

www.cnet.com/how-to/how-to-do-a-factory-reset-on-android/

Learn how to perform this "last resort" method on your Android.