How can I prevent external hard disk from sleeping when connected to Airport Extreme? I have a 1TB external hard disk from Seagate connected to the USB port of my Airport Extreme and everytime I need to access it I have to wait some time until it leaves the sleep mode. 

Is there a way to prevent it from happening?
You can run Keep Drive Spinning on a Mac that's connected to your drive over the network. It continuously accesses a hidden file to keep the drive awake.

You can also set it up yourself per these instructions.