Prior to the invention of microprocessors, computers were built in mainframes with a backplane that had slots for connecting components. The backplane generally resided against the back of the computer case, which is how it got its name. Some systems used rails to ease the daughter boards into the slots. A backplane is generally favored over cables because it is more dependable and does not need to be flexed like cables do each time a card is added to an expansion slot. Eventually the cables wear out from continual flexing. The lifespan of a backplane is correlated to the longevity of its connectors. Backplanes are also used in servers for storage devices. Hot-swappable storage devices can be removed from the backplane and replaced without shutting down the system. In addition, backplanes are used in disc arrays and disc enclosures for power disc drives.