What is the best solution to backup customer's sites? Can second HDD help?

IMHO an off-site backup solution is much better. Sure with a second hard drive you save bandwidth charges but lets look at several scenarios.

Bob the terrorist decides to bomb your colo facility and your main drive as well as backup are gone.

Jimmy the could be system admin goofs and the network goes down and it's not the best host in the world so they said it could be a week downtime.

Martha the cracker thinks she's how they call them '733T hAxx0r' and whatnot... and breaks into your site and destroys anything site.

In the previous three situations, if you had a second drive in the server for a backup... you really wouldn't be able to rely on it too much.

You may be better off getting a cheap $99 RaQ somewheres and get an automated backup program and transmit the backup files to it daily, or however you schedule it.

Regards,

Mike