said by antdude:... I finally tried it. Full Erase failed: »i.imgur.com/NWEnF.gif but Full Overwrite worked. Recuva couldn't recover any files so I hope that' secured enough.
A full overwrite should work for normal purposes. The problem that Kilroy
was referring to is that in a flash device with wear leveling, there is a built-in mechanism that assures that all cells in the device get written-to roughly the same number of times, to prevent wear-out from literally writing/erasing/writing too often to the same cells. In such a case, any kind of partial rewrite to the device may skip over a previously-used (but marked as "erased") cell in favor of a cell that's been written-to fewer times. A forensic access of such a device can conceivably look into those previously-used cells and read what voltage states are still actually left in them. By fully writing to the device, all cells are over-written with the voltage states corresponding to the new signals.
That still leaves the question of whether an extremely sensitive, scientific lab analysis might still be able to see a slight variance from nominal in a cell's present voltage state, left from the data that was there prior to a single re-write... but the likelihood for success of that kind of forensics is very questionable, even among experts - unless the over-write signal is all ones or all zeroes. In that case, the faint 'echo' of the cell's stored voltage state that was there before may conceivably be slightly easier to detect and "read".