Comment 9 for bug 1512857

Actually, raid isn't built in; there are raid1, raid10, raid456 modules. Testing if I get that message when the module isn't loaded and maybe somehow the module wasn't loaded in your case (or loaded too late)?

No, mdadm will load the modules for you. If I move them out of the way, mdadm fails with:

mdadm: RUN_ARRAY failed: Invalid argument