external drive will only connect directly plugged in and not through usb hub

Is this external USB hub self powered ... or bus-powered? The latter may not provide a sufficient amount of power to this drive when used.

... and the same for this external drive? If so, then it may just be that this hub/drive combination is not compatible. I am assuming that:

1. You tried connecting this drive to each of the hub's ports.
2. You tried connecting the hub to each of your MBA's two USB ports.
3. You tried using this hub/drive combo with another computer.
4. This combo worked before with this notebook.