I want to sell an old Android phone. How can I delete sensitive data on my Android phone with a MicroSD card from within Ubuntu, in a secure manner?
You can erase SD card using a command line tool called shred (manpage)
You can also refer to this discussion in askubuntu.
How to delete file(s) in secure manner?
OR
Here is a tutorial (but you don't need live cd, if you are on ubuntu) - http://www.howtogeek.com/howto/15037/use-an-ubuntu-live-cd-to-securely-wipe-your-pcs-hard-drive/
For deleting everything on phone, Hard reset is OK. But I recommend flashing the original phone firmware. You can get instructions at XDA
