The Apple TV 1 as a back up device.
Other than content purchased through the Apple TV, there is no way to move content from an Apple TV to an iTunes library on a Mac or PC. This removes the possibility of using any synced content held on the Apple TV as a back up or replacement for content which for one reason or another (e.g. Hard Drive failure) may have been lost from your iTunes library on your Mac or PC.
You should never think of the Apple TV as a back up device for your iTunes content.
The Apple TV 1 as a stand alone device.
The Apple TV can in some circumstances work as a stand alone device and indeed a small number of users have used workarounds to copy content from their iTunes library to the Apple TV and then remove that content from the iTunes library on their Mac or PC.
Unfortunately, the circumstances that make this possible also make it all too easy for user error to erase the Apple TV and thereby lose all content.
Unless you fully understand and accept the potential consequences of using the Apple TV in this way it is generally suggested that you don’t.
Purchases made via the Apple TV 1.
Purchases made from the iTunes music store directly to the Apple TV are by default transferred to your paired iTunes library on your Mac or PC when iTunes and the Apple TV are configured correctly.
This action is by design and intended to protect your purchases against loss. Generally speaking you should not attempt to stop this occurring.
Think of the Apple TV as an iTunes extender rather than a stand alone device.
Allow purchases made via the Apple TV to transfer to your Mac or PC as soon as practical.
If space becomes limited on your Mac or PC as a result of your growing iTunes library, use an external drive (a USB, FireWire, Thunderbolt or eSATA drive is best but a network drive can be used for this purpose) for some or all of your iTunes content. Use the articles suggested below to learn how to do this:
Instigate a regular back up procedure for your iTunes library (and indeed any other files you consider valuable) and keep at least one (preferably two) back up(s) of your library.
Don’t forget that photos are not part of an iTunes library and should also be backed up to protect against future loss.