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.
With the sand ready, we select a flask that will hold our hard disk with room to spare. First, coat the hard disk and molding board with parting dust so the sand doesn't stick to them. We pack the bottom half of the flask (the drag) first. Place the hard disk upside down on the molding board, and place the drag around it. Add loose about 2 inches deep, pack the sand with the ramming hammer (be firm!) and repeat until the drag is slightly overfull. Scrape the surface clean, starting on the near side and working away from you in several steps to get a clean surface. Sprinkle some loose sand on the scraped surface, and rub in the bottoming board. To do this, press down firmly, and rub the board back and forth slightly. This allows the loose sand to fill any cracks or other imperfections in the bottoming board, so that the mold will be firmly supported during the casting process.
The next step is to pack the top half of the mold (the cope). Take the assembly of molding board, drag, and bottoming board, and flip it over so the molding board is on top. Remove the molding board to expose your hard disk. Remove any sand on top of the hard disk. Sprinkle a fine layer of parting dust over the disk and exposed sand; this will prevent the two halves of the mold sticking together. Wipe off any excess. Then, place the cope over the drag; alignment pins should accurately locate the two halves. Pack the cope in the same manner as the drag.
Next, separate the two mold halves. Before removing the disk, tap it gently with a hard object to loosen it in the sand so that it will pull out cleanly without damaging the mold. Hard disks have very shallow angles, which makes this a very important step. Even so, pulling the disk out of the mold will be tricky. There are a variety of ways to do this; I went with the simple method of just grabbing the edges of the hard disk cover with my fingernails and lifting the disk out. This was less than ideal, as the edges of the mold crumbled slightly. If getting a really clean casting is important to you, you may want to try something more complicated. In my case, I was happy enough with the result. If things go poorly, you can always repack the mold and try again.
Stuff to watch for: lift the part straight out without shifting it sideways. Be sure to use plenty of parting dust when packing the mold originally. If your sand is too dry, it can get very crumbly at this point. Don't be afraid to just try again if needed; packing a mold is quick, and a lot of novice casters try to salvage molds that they would be better off just repacking from scratch.
If there is any loose sand in the mold from parts that broke off or crumbled, remove it. Don't try to pick it out with your fingers; you'll make the problem worse, not better. Instead, flip the mold upside down so the sand falls out, or simply blow small quantities of loose sand out.
The mold is almost finished, but it still needs a place to pour the aluminum (a sprue). Cut a hole in the sand in the cope from the center of the mold out to the surface. There are many different ways to do this; I did it by gently pushing a pieces of 1/2" copper pipe through the sand. This is most easily done with the cope sitting on edge. Next, carve a pouring basin centered around the sprue. This needs to be large enough to provide an easy target for pouring aluminum from the crucible. A butter knife works well for carving this shape. It should be about 3" in diameter, and relatively shallow. Exact details are not crucial. After cutting, remove any loose sand, and press the surface gently with your fingers. This packs down any rough spots, which helps prevent any sand from being washed into the mold when you pour.
The mold is now complete. Reassemble the cope and drag, place the mold by the furnace, and get it close to level for best results.
It's now time to melt some aluminum. Inspect your crucible, load it with some aluminum scrap, place it in the furnace, and light the furnace.
You can place your hard disk in the crucible now, but I prefer to add it to an already hot crucible -- it's more fun that way! You'll need additional material anyway, so you might as well start with some scrap. Scrap castings are best; hard disk cases work well, soda cans are ok if somewhat messy, and scraps from the machine shop scrap bin at TechShop are also good.
Dirty scrap like hard disks makes copious quantities of nasty smoke. Stand upwind, cast in a well ventilated area, and avoid nasty fumes from the furnace.
Once your aluminum is hot, clean off the dross and pour. Remember, make sure you have clean, shiny, fluid metal before pouring. Never pour with anything amiss. Pour ingots with any excess, and return the crucible to the furnace to allow it to cool.
Once the casting has cooled, remove it from the sand. Clean it up with a bandsaw and disk sander. Remember to use appropriate precautions when sanding -- you don't want to sand any casting sand stuck to your part. That can cause silicosis and other respiratory problems. Be certain you have all the sand off first.
Make it look all spiffy. I used a sanding disk on a right-angle die grinder to smooth and shape the sprue, which I then used as a stand. I didn't cut or sand any of the mold flash, since I was going for that "as-cast" look. (Hard disks are a tricky pattern, so there was a lot of mold flash.) Make sure your sprue is smooth enough not to scratch up whatever you put it on.
Congratulations, you're ready to enjoy your brand new solid-state hard disk!