Most of the times if you just need to make a drive backup
sudo dd if=/dev/disk# conv=sync,noerror bs=64k | gzip -c > <ouput filename>.img.gz
if you need trottling because you have a bad disk drive
sudo dd if=/dev/disk# conv=sync,noerror bs=#k | pv -L 10m -s 500g | gzip -c > <ouput filename>.img.gz.img.gz