You could always do this. In fact, window's kernel does this with pages(Address spaces or in simpler terms, processes). When pages are inactive they are flushed to the harddrive, and when they're needed they're reloaded into the RAM. The only problem is, even with a direct connection to a Harddrive, this process is slow as fuck if there is constant flushing and loading, which there probably will be if you intend to play games that need more RAM then provided by your computer. The reason for this is because opposed to sending and receiving signals through the BUS immediately with little delay from the RAM, you have to wait for the disk in your harddrive to do a full rotation. Which could takes a lot longer then the RAM would. On a flash drive, it will be a lot slower then the harddrive. Ultimately, it would depend on the read\write speed of your hdd\flash drive, but even some pretty fast ones will cause it to be quite a slow process.