Have you tried toggle data/media in "backup & restore" section before formating data? This will totally format your data partition including internal storage known as /sdcard/.
HTC Desire 601 UL
Asus Nexus 7 2013 (flo)
Acer ICONIA TAB A500 16Gb
[TO Omni Rom Kitkat v13]
“Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine.” -Nikola Tesla