You can recover from flash devices from slackspace anyway. I suppose due to the nature of the technology the data will persist until it is overwritten.

Low level formatting and rewriting should take the flash devices beyond reasonable recovery. I don't know if it's possible (even in theory) to recover overwritten data from flash.

CD/DVD considering the cost of disks, if you want the data gone shred them physically or scratch the top surface down to the dye. I wouldn't mess about with formatting.

I suppose it could be possible to detect subtleties in the dyes of CD/DVD after formatting. Black helicopter time there though.