How-to: Upgrade the drive in your Apple TV - Engadget
www.engadget.com/2007/03/23/how-to-upgrade-the-drive-in-your-apple-tv
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
We decided to double the size of the HDD from 40 GB to 80 GB; we picked up a new 5400 RPM IDE laptop drive and went to town.