Accessing the hard drive is fairly easy on the TAM. There are two tension clips near the bottom of the back which will let you lift off the cover (this is shown in the logic board removal guide). At this point you are basically looking at the drive. It is mounted horizontally, on top of the processor heatsink. It has a small assembly with a fan that is held on with four Phillips head screws. Remember to remove the fan connector as well. The drive is just a standard issue IDE laptop drive. You can find one in most late 90s and early 2000s laptops. Booting from a system disc will let you initialize and format the drive. Even if it's 40GB your Mac will likely only see 4.5GB (this is normal and there is more nuance on this topic). The screw holes might not line up on the bracket so you may need to be a little creative. But as long as the pins align you should be in business for very low $$.