[Formendacil]

I would say that the Dwarves MUST have KNOWN of Erebor...

But perhaps they did not know about the caverns... Perhaps, Thrain's discovery was the large, inhabitable caverns already waiting in Erebor to be settled- perfect territory for the Dwarves, who would have been looking for a ready-made home, I imagine, after losing both Gundabad and Khazad-dum. Maybe Thrain took a look around and said:

"Yup, this looks good. All we need is a little dynamite, to make a decent-sized door over by that river there, maybe a few power-sanders to smooth things down, and perhaps a few Men to grow some cabbage, and we're all set. And hey! Is that a really big, shiny, Arkenstone over there? Yup, my Dwarvish heart says this is perfect."

Or something like that...