This is what GNU shred does. It has a high speed PRNG meant to avoid performance issues from directly using /dev/[u]random. Point it at your preferred block device or partition and let it fill your drive with entropy. With modern flash devices there is still the issue of hidden reserved space for block replacement but you will cover most of the drive. Reduce iterations to 1 since the default is meant for legacy hard drives and will needlessly add erase cycles to flash.