I'm looking for a tool that allows me to clone my main hard drive (/dev/sda) into a secondary drive (/dev/sdb) without having to unmount the source, because the source is live and cannot be unmounted, it has to be a perfect copy since the secondary drive will be used as backup of the first, so in case something happens to the first, I can boot from the second one and revert the process (clone /dev/sdb to /dev/sda)
I tried using dd, but after it finishes, I check /dev/sdb and it shows used 0% and it won't let me mount it to see if it has the same partitions as /dev/sda.
Thanks.