When disposing of an old hard disk, it might be tempting to simply throw it in the garbage, or sell it. However, first you should remove any sensitive data from the disk. This might be financial data, old email, login information for web sites, and the like. Unfortunately, simply deleting files doesn't normally remove the data -- it just marks the space as available and removes the file from the directory listing. The data can be recovered at this point, often quite trivially. If you wish to protect your data, you need to do something more thorough. There are some software utilities that can do a fairly good job, but if you're feeling especially paranoid (or simply want to have fun!), a physical process is required.
Please note that casting is a potentially hazardous activity. Observe appropriate safety precautions. The details should be covered in whatever casting training you received, whether online or in a class.
In this instructable, we'll erase the disk by melting it. Hard disks are mostly made of aluminum, which can be recycled, so don't throw it out! Instead, we'll cast a new solid-aluminum hard disk using the original as a pattern. For this project, we'll use the aluminum foundry at TechShop Raleigh-Durham. That way we have all the casting tools we need, including sand, furnace, flasks, and molding tools.
The first step in any casting project is to prepare the casting sand. Start by sifting the sand to remove debris and break up clumps. Then add water to temper the sand so that it sticks together. Add enough water to make the sand slightly moist, but not wet. It should clump when you squeeze it in your hands, and break cleanly rather than crumbling when you break it. If you add too much water the sand will get sticky, and stick to both the tools and your pattern, making it difficult to work with. The additional water will also generate more steam, which can interfere with the quality of the casting.