Yes, I did read them. If I'm not mistaken, the second one talks about create an array between two non-os disks, and the first one (I quote)
In Disk Utility, just access “File->Create->Raid Array…” on the menu and choose the options. Before doing that, you might want to clear off the drives you’re going to use (I generally create a fresh GTP partition to insure the drive is ready to be used as a component of the RAID array).
Apparently I have to erase disks I'm going to use with a new partition, and I can't obviously do that with a system disk...
By the way, I have mdadm installed, althought maybe I'm not so good with it.