I don't know how old your old piece of junk is, it might be a lot newer than some of the systems that I still run. In fact, that it can run Knoppix without a hard drive swap file confirms that it is. But yea, it's generally a bad idea to upgrade old hardware. And depending on the age you might even find new hard drives will not work with it (many older systems had serious limitations on the size of hard drive they will support). But, if you don't have a spare you may be able to scrounge on old hard drive for it at some point. Until then Knoppix should be fine without a hard drive.