I understand what you are trying to do. I have pondered the same issue.
why apple tv does not have a usb input to plugin an external disk that can be accessed throughout the home network is beyond me. Greed, Control, and Arrogance come to mind but that is another issue...LOL
The only way to stream from your external disk using apple tv is by complicating your life with itunes and have your computer running 24/7 just so that iTunes can access the library which is stored in the external drive anyway. ridiculous.
My work around so far is to use the WDLive TV. it has all the basics as the Apple TV, Netflix, hulu, etc..PLUS 2 USB inputs that can share media in an external drive to your devices and the WDLive TV box does NOT have to even be turned on. (except when you want to watch your media on your tv obiously.)
I have a 2T disk with all my media, connected to the rear USB, and a wireless keyboard with the receiver connected to the front USB input. I sit on my couch, power up the boob tube, (TV) and navigate my little heart away using the keyboard. Also, The WDLive TV does not make you feel like someone's hand is in your pocket like AppleTV does.
another workaround, minimal expense involved but worth it. an atom mini-pc, you know, those little useless pcs that may find a place in this application, Install itunes, ext drive attached to it. point your itunes library to the attached ext drive, leave it on, close the the lid and tuck it somewhere in your cabinet. better than having a desktop with a 750 watt power supply on 24/7.
another good option, router with usb input. your hard drive will be available throughout the network. nothing but the router needs to be turned on, and your ipad can access the media with such apps as OPlayer, LuberPlayer etc..
One positive note on the apple tv is, you can't beat the navigational performance.
Other than that we have to recognise that, just like all apple products, it too, is a consumption product.
Good Luck in your own search for an answer.
Lou