Depending on the selected operating system image the 2nd hard disk can be used as extra memory.
Image with Raid1:
The 2 hard disks are configured in the Raid1 compound. Both hard disks contain the same data and compensate the failure of one disk without loss of data.
Image with Raid0:
Both hard disks are recognized as a big disk array by the system. Here the disk size is (almost) as twice as big than Raid1 if both disks are the same size. WARNING: Failure of one disks leads to a complete loss of data!
Image without Raid:
The operating system is installed on the first disk, the 2nd disk (/dev/sdb mostly) is deleted and available after a manual formatting and preparation. The 2nd hard disk can then be mounted on its own.