use Dariks boot and Nuke.
Seconded. Free, works perfectly well with a variety of wipe options and is accredited to US DoD standards (althought at the end of the day, that accreditation doesn't really mean anything, secure wipe is secure wipe).
However, an important detail to note which others haven't mentioned is that a bootable bios-based utility is
required - see below for the rationale:
1) In order to securely wipe a drive,
all sectors must be overwritten.
2) All drives (HDD/SSD etc.) come with a certain amount of 'spare sectors'
3) These sectors are used by the disk's internal controller firmware to 'swap-out' defected sectors for spare ones.
4) The 'broken' sectors are then marked as inactive by the firmware
5) Operating systems ask the disk for active sectors to store data on, the disk controller suitable obliges
6) After a period of time, several of these bad sectors do have recoverable user data on them (and not in small chunks either, a remarkably large amount can be recovered this way). Note: on SSDs and Flash-based media this swapping takes place as standard for load balancing (avoiding repeated-write defects) and as such huge amounts of past data will be recoverable
7) An OS based disk wiping tool will not be able to access these sectors as the drive controller does not present this information to the OS, only the conventional filesystem
8) As a result, the OS will never be able to overwrite these sectors and information will be leaked
9) A BIOS-based utility can use lower-level hardware instructions to ask the drive controller specifically for access to these sectors - which exist
below the 'filesystem'
10) This will allow recovery and/or secure wiping of these sectors and hence the entire disk
I hope that made sense, feel free to reply/PM if you have questions.
- As for the original post, flash drives have a pretty non-existent second-hand market (except criminals looked to harvest details as outline above from non-tech savvy people), so just smash it up when it's no longer required.
