Securely erasing data partition?

[newbie455]

I have a Droid 1 that I want to sell but it has data on the internal memory that I can't risk being stolen.

I did a "factory reset" but I cannot tell if that is a secure erase or if it just formatted the internal data partition. Does anyone know if Android actually overwrites the data partition when you do a factory reset?

If not, how can this be accomplished?

Thanks