Making music and photos available to Apple TV
This is similar to another recent question: Is Apple TV intelligent enough to stream directly from a NAS?

As @tubedogg says, you need iTunes running on a Mac/PC in your network to share local files.  You can only remove this requirement if you buy/stream everything from the iTunes store.

Unless you hack your ATV.

You can run something like PlexConnect on it to stream content from a local Plex server, but again you need a Mac/PC running a Plex server, so it's not that different than requiring a Mac/PC running iTunes.

I think there might be an XBMC hack (for ATV2 only) that will allow you to stream from a NAS/UPnP device, which might work with your modem's USB port, but I doubt it will work from the AE's USB port.  The AE's USB port is best used for a Time Machine drive, which is a great backup strategy, but off-topic.

So, if you haven't bought everything from the iTunes Store (or haven't used iTunes Match to make a copy of all your music available to you from the "cloud"), you do need to keep a Mac/PC running iTunes to access your personal content, regardless of where your USB drive is plugged into.