Snow Leopard hangs on grey screen when usb hard drive attached

I recently installed Snow Leopard on my macbook. I bought a fresh HD 320gb and installed it in my mac and then clean installed Snow Leopard on it.

Since installing SL, when ever I have my two external USB hard drives attached to my macbook via a USB dock and boot my mac it hangs on a grey screen for a very long time usually on the spinning circle. It never gets to the Apple logo or the login screen. As soon as I disconnect the HDDs or shut the machine down and reboot without them being connected via USB my macbook boots very quickly as expected.

Any ideas what might be causing it. The Hdds themselves or the hub and any solution to the problem? This never happened on 10.5 just seems to be occuring since upgrading to 10.6 Snow Leopard.

Thanks

Is the USB port (or dock) a mains powered unit?

Sounds very much like the externals are taking too much of the power from your macbook if they are powered from the machine.

Yeah it's a mains powered hub. It's a Belkin 5port iPod/USB dock. This one specifically;

Belkin USB 2.0 5-Port HUB W/Ipod Dock: Amazon.co.uk: Electronics & Photo