Backup your original Hard Drive
The Phoem method may be superseded by Engagets. Please consider doing a complete ~40GB backup of your original drive.
Phoem's Fast DD Method (OS X)
This will allow you to make copies of your AppleTV system partitions in case you need to restore them to a virgin state. This has only been tested on MacOS X.
NOTE: These instructions assume /dev/disk2 is your AppleTV drive.
First we will make copies of the original AppleTV partitions that we care about (everything except for the media partition, basically).
Lets copy the partitions onto your local drive:
dd if=/dev/disk2 of=Partition.img bs=1024k count=100
dd if=/dev/disk2s1 of=EFI.img
dd if=/dev/disk2s2 of=OS.img
dd if=/dev/disk2s3 of=OSBoot.img
Now lets put it back in place onto the new drive:
dd if=Partition.img of=/dev/disk2 bs=1024k count=100
dd if=EFI.img of=/dev/disk2s1
dd if=OS.img of=/dev/disk2s2
dd if=OSBoot.img of=/dev/disk2s3
Now use iPartition demo to find out the size if you were to grow the partition completely; using up the free space.
Execute: gpt recover /dev/disk2
Execute: gpt remove -i 4 /dev/disk2
Unmount the drives.
Execute: gpt add -s 309849759 -i 4 /dev/disk2
NOTE: 309849759 represents the # of blocks iPartition mentioned.
Finally, open diskutility and format just the media partition. (erase)
XXX represents your AppleTV 2.5" notebook drive. It could be sda/sdb/sdc (USB) or hda/hdb/hdc (IDE) depending on your system, and how you connect the drive.
Execute: dd if=/dev/XXX of=appletv-full-disk.bin bs=1M
This will create a full ~40GB backup of the drive. In the future, we will have to work on manipulating the partition table under linux, given HFS+ support is missing.