By "raw read" just means: "read the bits and don't bother with error checking or correcting". When you copy files from the disc, the whole mechanism, including error correction is fully at play. Raw reading would be necessary for any statistical method to even be possible. Then you'd have to do the error correction yourself - and be faced with the same problem again. You don't know, which bits are bad. Statistics don't help - many of those bits will persistently be flipped the wrong way (the error correction mechanism will fix them, but only if.... ah, well I already explained that).