Water damaged Diamond. How to clear internal storage before returning to Asurion?

[exi]

I have a replacement for my insured Diamond en route. I managed to drop mine into water.

The touchscreen is no longer functional; neither are the "hard" buttons, save for power/sleep on top. I have flashed a stock ROM to clear any information on the device, but how might I go about clearing the internal storage of the phone before shipping it back to Sprint/Asurion?

I forget whether there were any things of importance there. The phone is not showing in the device manager, but the USB cable seems to have been enough to flash the phone.