I have 4TB disk (A) with data in it. Now I've bought a new 4TB disk (B), and I want to start a RAID1 so that B becomes the mirror of A, without loosing data on A. I'm under linux with mdadm tool.

The only guides I've found by googling are referring to a typical situation where both A and B must be initialized and formatted. 

Is there a way to do this?