Downloaded and installed the Beta, made sure the UnsafeDirectIO thingy is still "True", rebootet the server, still nothing.
Switched the USB drives around a bit and am now sure that it's not the Hub or any specific port (suspected maybe different controllers): previously recognized drives are still recognized through the Hub and also through the same ports I tried for the news drives...
Soooo, what's next?
Anyhow: thanks for the quick feedback :-)