Problem almost solved: I changed from a usb3 copy machine to a usb2 copy machine and the disk was copied successful.
Actually I have two hypotesis: 1) the source disk has some physical problems but this doesn't explain why fsck_ffs doesn't detect any failure. 2) As I'm using 3.5'' disks too, their hotness have burned somehow the usb3 copy machine board or circuit. I'm doing the backup from the new copy and test the usb3 copy machine job again now. If you have anything to add don't be shy.
