Also, some (flash) drives are compressing data on the fly. Writing zeroes to such a drive might just not actually write anything at all to the flash cells excep

Also, some (flash) drives are compressing data on the fly. Writing zeroes to such a drive might just not actually write anything at all to the flash cells except for some run length descriptors (numbers). Since the worry seems to be speed, here's a tool I wrote to get random bytes about as fast as drives can accept (well, at least spinning disk drives, there might be some flash drives that are faster): https://github.com/pflanze/fastrandom https://github.com/pflanze/fastrandom I'm expecting you to figure out where this tool goes wrong and produces predictable bytes yourself. Also, please tell me if you do :) Anyway, even if it isn't secure, it will be enough to foil compression algorithms.