The only reliable ways to ensure no data can be retrieved from a flash memory device are:
1) only ever put well-encrypted data on the device. "well encrypted" means at least AES-128 and at least a sixteen-character pass phrase, the longer the better.
2) completely destroy the device. Only two effective ways: use a hammer or equivalent to reduce the chip(s) to grain of sand size; or chuck it in a really, really hot fire - think crematorium or scrap metal man's furnace
This was official advice from an agency whose job it is to know