recoverable

My friend is selling his hard disk and he wants to delete everything on it. Which software can delete data, which can't be recoverable ?

Hi Guys! How can I erase unwanted data from a multi-session CD/DVD. Normal erase method is simple, just deleting the data when making a new session, but that data is retained in the CD/DVD and is always recoverable. What can be done so that the data we erase cannot be recovered by any means...