why averaging only 4MB/s and how to spare bad blocks?

[dnanian]

Steve: we don't use either ditto or rsync -- our cloner was custom designed and written for speed.

Certainly, when I copy large files I don't get 2MB/s -- I get much more, and slower performance when copying smaller files. I can't explain why your other was going so slowly, but yes, it could have been doing retries at a lower level.

Smart Update will take the same amount of time as an erase-then-copy if the drive is empty.

Our code/pseudo code is not available, sorry!