so what kind of minion would that be? has anyone made any stats? Screamer with machine trait? what kind of skill would you need for riding that? Operate (Flying Demon-Disc)?

I'd say a Screamer would be apt, just replace the Fear 2 trait with the Machine trait. If you still wanted the fear factor, i'd say make it +1 to your own Fear rating, that way it's not too unbalanced.

As for the talent, either Operate (Aeronautics), or if your GM wanted to make it that much more special, then yeah, having it's own Operate talent would seem to fit.

Riding living creatures was covered under Wrangling in earlier lines, which became part of Survival, right?

Seeing as the Disc of Tzeench is completely slaved to its master's will, and doesn't usually attack, the Minion rules seem inappropriate here. As its primary function is as a mode of transportation, I would instead treat it as a piece of wargear akin to a Jump Pack. Then the character would have perfect control of its motions, it couldn't fight on its own, it would need its own Operate Talent to fly it, and could not be separately targeted (except perhaps by a Called Shot, which could simply disrupt its connection to the material world, causing it to 'phase out' for a time rather than being destroyed).

I don't remember: Wasn't the disc included in the apocrypha?

Would you operate such a disc with willpower or agility?

I'd say willpower. And you'd take penalties to it if you're doing other things that concern willpower, such as channeling powers.

Doomaflatchi said:

They have a mount stat line in WHFB, and makes attacks like any other mount. I assume your just use to WH40k were mount and rider have a single simplified stat line.

Disks of Tzeentch are still a deamon in it's own right. But I'm also going to point out that the Disk is not very good at attacking things.