It appears to use hardwired PRNG. Admittedly it's a stretch to imagine flash manufacturers adding f3 defeat feature to firmware, but it's possible. I prefer using bulletproof AES data like:

     1. yes | openssl aes-256-cbc -e -out /media/drive/testfile
     2. eject /media/drive
     3. physically reconnect and mount the drive
     4. openssl aes-256-cbc -d -in /media/drive/testfile | grep -q '[^y]'