This is an important article on ensuring electronic client files are properly erased from storage media. It provides a comparison of different software programs and their effectiveness on deleting ESI.
"With stories abounding of identity theft aided by information lifted from discarded storage devices, you want devices you no longer plan to use to have no usable information when they head out the door. Here's how to wipe them clean.
Why Erasing Files Is Not Enough
Sure, you could erase the contents of the drive, but keep this in mind: the act of erasing a file does not remove it from a storage device. . . . " (Read more).