In most cases when a new drive is connected to the system Time Machine will prompt you for the setup. If this does not happen or if you are changing the backup drive, click here.
Any disk attached to your computer that can be used to store backups is listed. The list includes internal hard disks except for your startup disk, and most external disks.
When Time Machine performs the first backup of your system, it may take a while. You may want to set up Time Machine in the evening so that the initial backup can be done overnight. Subsequent backups take less time because only items that have changed are backed up.