[luser001]

I do something similar. You will need a powered USB hub to attach the external HDD (built-in USB can't supply the full 500 mW that a USB port is supposed to supply). Checkout out Plugable's videos on youtube.

I have a Model B with a connected HDD that's been on 24x7 for a few months now. No problems so far. I don't expect a big difference between the Model A and B wrt stability.