I don't think anyone have ever demonstrated the ability to recover data after a few overwrites on any modern harddisk. The entire concept of "residual" magnetism that you can reliably measure sounds ridicules to an engineer.

AFAIK no-one has ever demonstrated the ability to recover data after a single overwrite. Someone offered a prize for such recovery, back in 2008 [1] which was never claimed (although the prize was only of symbolic value).

[1] https://news.ycombinator.com/item?id=117588

I don't want to sound like a conspiracy theorist, but if I was a 3 letter agency with infinite resources (so also no need to claim any prizes, no matter how much tempting they can be) and succeeded in recovering data that way, I would never tell anyone. Doing so would reveal what I can do, and by extension send the message "next time do a multiple pass wipe" which in the end would reveal what I cannot do.

Considering that brilliant physicists and engineers have private access to state of the art technology, I find it highly unlikely that NSA/FBI/CIA/??? have figured out how to do this, but no one else have.

On Linux this will generally be CPU bound instead of IO bound because urandom is very slow. It's much faster to mount it as an encrypted drive with a random key and then dd over the encrypted mapping.

Eg:

cryptsetup open --type plain --key-file /dev/urandom --key-size 256 $DEV crypttmp

dd if=/dev/zero of=/dev/mapper/crypttmp bs=4k