Just prepare a 512-byte buffer with all "U" (uppercase letter U) and write a serial number on TOP of it starting with 1. Then dump that buffer, incrementing the serial number as you go, until the disk is full. 1) Flash disk compression will fail because of the serial number, so all "spare areas" will be filled. 2) The "U" is a series of alternating 0s and 1s - very high hysteresis for magnetic drives. 3) Anybody assuming that "deletion" is only removing directory entries will be unpleasantly surprised. Very easy with Linux and C.