Hi,
it's not easy to say what really happens but as you mentioned about the smell might be somethings burnt inside of laptop. Theoretically, you can remove hdd and try to enter bios if you will be able to check if hdd recognized. maybe, there was short-circuit in the system when the external hdd was connected ( Sometimes, it can happen.........). Anyway, if it's under warranty, you should contact the asp (service center). but don't tell them that it happened connecting the external hdd
