Hardware can go bad and connections can loosen over time. I've had drives disappear only from the cable coming loose. Unplugging and plugging back in solved the issues. My current desktop is only report 48GB RAM when there's 64 installed. Undoubtedly at least one of the 8 DIMMs has become unseated or gone bad, but I've been too lazy to troubleshoot because I don't use that much currently.