Unlike magnetic devices, flash usually don't leave any significant traces of previously recorded data. Writing new data usually is page erase - page write cycle. So, really, it does not matter what data will be written. So, fast urandom is good enough. To be paranoid you could write it twice.
There is another thing in flash-based devices that could be potentially dangerous. Flash devices usually have some spare memory to use instead of failed blocks. And once internal processor finds bad block it just replaces it with spare one. And data recorded in bad block still stay there and can't be erased by usual means. From the other side, if flash begin to fail, than it fails fast and become unuseable. So it makes no any sense to keep such device at all and it could be just crashed or microwaved to destroy all data,
/dev/urandom is a source of pseudorandom sequence, /dev/random is "true" random so /dev/random could be slower.
I think for erasing flash-based devices like SSD, USB-sticks or flash cards you could use /dev/urandom with same efficiency.
Unlike magnetic devices, flash usually don't leave any significant traces of previously recorded data. Writing new data usually is page erase - page write cycle. So, really, it does not matter what data will be written. So, fast urandom is good enough. To be paranoid you could write it twice.
There is another thing in flash-based devices that could be potentially dangerous. Flash devices usually have some spare memory to use instead of failed blocks. And once internal processor finds bad block it just replaces it with spare one. And data recorded in bad block still stay there and can't be erased by usual means. From the other side, if flash begin to fail, than it fails fast and become unuseable. So it makes no any sense to keep such device at all and it could be just crashed or microwaved to destroy all data,