If a storage device is connected to a Mac via USB or Firewire, you can eject it so that it disappears from your Mac. This post explains how to remount it, without unplugging it and then plugging it back in. This is mainly of interest if you use an external (non-SSD) hard drive with Time Machine: After a backup, if you don’t want it to make any noise, you can eject it and it spins down. Then the problem is how to remount it for the next backup.
It is quite simple: After ejecting a storage device, open Disk Utility. It should still show it in its sidebar. Now you can use the button in the toolbar to mount it.