Unless you have a way to lock the HD, I would say it's probably not a good idea.

i'm bringing an asus eepc on my upcoming trip. it shouldn't have any problems with the bumping but i will let you know...it is going in one of those bike friday suitcase trailers :p

I've put 300 miles on my bike with my laptop in a pannier bag.

No problems at all. And my bike ride is mostly a rugged, bumpy bike trail, and my mountain bike has no suspension.

I do try to pad the laptop with clothing to keep it from jostling around in the pannier, but otherwise I take no special precaution, other than to turn it off first, obviously.

It does probably help to have a durable laptop. Mine is Dell business class laptop and the hard drive is more rugged than those found on a cheaper home version laptop.

I think the bike ride is easier on my laptop than going through the airport. Only time I have ever broken a laptop was at the airport.

Here's my suggestion if you are worried about your laptop.

Buy some foam, like the kind used in camera and gear cases, or even better for absorbing vibration, the softer cheap foam rubber like the kind in cheap mattresses and pillows. Make a cut out so that your laptop will fit in the foam. Use this foam to encase your laptop in the pannier bag.

This should minimize vibration and jolts to an acceptable level. As long as your pannier doesn't take a direct hit from a collision or a crash, I think it will be safe if encased in foam that absorbs the vibration and bumps of the ride.