[Bug-ddrescue] Copy speed

[David Morrison]

Just as a data point in copying speed, I copied a 1TB disk twice on a 
Mac Pro with the disks internal to the case.

With -c 128 and block devices (/dev/disk), transfer speed was 37MB/sec.

With -c 256 and raw devices (/dev/rdisk), transfer speed was 110MB/sec.

So definitely worth it to cut copy time. If using external disks, 
there may not be such a difference.

Thanks

David