On the front of your case, you may have a "blank", to put in another 5 1/4 drive. Instead, you put in a special receiver (like a trailer hitch), that accepts a hard drive mounted in a "drawer". You slide the drive in when you want to use it, and take it out for storage. These are actually designed for "servers" which are "hot swappable" (you can do this with the computer on). Used for security. On normal desktops, you have to shut down to put in or take out. You can have your normal OS, with all your programs on one drive, and another drive with just OS and your a/v programs.