With the switches set to JBOD or Just a Bunch Of Disks, each drive appears as a separate disk available to the operating system. Here in Windows Disk Management we see both the 1TB and the 80GB drives listed as available to create a partition on. Once the volume is created, Windows will assign a drive letter to each which can then be accessed independently.
Changing the unit to use the BIG (Spanning) setting combines both drives to appear as one single drive to the operating system. In this screen capture of the drive properties, we see that the capacity of the "drive" is roughly 1080GB, equal to the combined total of both drives. BIG offers spanning only, unlike the performance striping done by RAID0 for example, which means that it will begin writing data to drive 1, and as capacity fills it will spill over to drive 2. If one drive fails only the data that was written to that drive is lost; the other can still be accessed.
Using the HD Tune read test to measure the performance of the drives, we see that the USB 2.0 connection limits throughput to an average of just under 33MB/s. While this is far short of USB 2.0's theoretical limit of 60MB/s, this is actually a pretty typical transfer rate compared to other external USB enclosures, which usually fall within 30-35MB/s in speed. Since the 2½" 80GB drive is much slower than the 3½" 1TB, we see the transfer rate drop off significantly towards the end of the test when HD Tune moves from the first to the second drive.
Let's wrap things up with some final thoughts and conclusion.