Saving files to external hard drive via WiFi

[Atreyu]

I have an iMac and a Macbook Pro. Both are running OS X 10.7. I have a WiFi network to which both computers connect the Internet.

Connected to the iMac is an external hard drive that I use for backing up files, and it's mounted on the desktop. What I've been looking for is a way to get the Macbook Pro to connect to the external hard drive for backup purposes as well, via the iMac using the WiFi connection.

(The wording of that last sentence looks funny, but I hope the meaning comes across all the same.)

Is there a way to make this happen? I'd rather not have to physically disconnect my external hard drive from the iMac and connect it to the Macbook Pro every time I want the laptop to access that drive. My Google-fu is not strong enough for me to find any possible answers to this.