Kind of a fork to the original post.
"if" tends to mean "input" and "of" tends to mean "output"
irgunII, jpollard says, and I will make stronger, don't boot to sdb and attempt to clone it with dd. You'd need a live state clone for that.
Boot to some live media and then clone it. Consider other ways like file by file. Gparted, clonezilla, redobackup and many more may be quite easy to copy your install.
Remember sda and sdb are very relative and can change so be careful. dd command is a very poor/dangerous way for a newbie to try a clone.