Just to add up to the (perfectly fine) existing answers:
Why do you need bs=64 (Blocksize)? While I cannot tell whether it is the fastest setting at all, the simple answer is, that it runs way faster (around 4x) than the standard settings... at least on my system. Tim Williscroft states that 100M could be faster, more research
could be needed was done here, have a look.
Test data here:
(I cancelled the first run because it took too long imho.)
$ sudo dd if=/dev/sdb4 of=/dev/sda2 status=progress
12962501120 bytes (13 GB, 12 GiB) copied, 394 s, 32,9 MB/s
$ sudo dd if=/dev/sdb4 of=/dev/sda2 status=progress bs=64K
13143113728 bytes (13 GB, 12 GiB) copied, 98,0026 s, 134 MB/s
status=progress is useful to monitor what happens, alternatives to that see this "Unix&Linux" post.
I would also recommend adding
conv=noerror,sync for error safety.
Also by Tim Williscroft: You could add the convention
What you could also look into is a trick by Mistiry that
Nobody seems to know [...]
dd is an asymmetrical copying program, meaning it will read first, then write, then back. You can pipe dd to itself and force it to perform the copy symmetrically, like this:
dd if=/dev/sda | dd of=/dev/sdb. In my tests, running the command without the pipe gave me a throughput of ~112kb/s. With the pipe, I got ~235kb/s. I've never experienced any issues with this method. Good luck!
While he seems to misuse the the word symmetric in sense of meaning, this would probably also be worth a try.
Comment by groxxda: "If you do this and specify a blocksize, make sure you use the same blocksize on each invocation." (The respective post is also worth reading)