Simsy:
Perhaps at the time the video was made, there were only ten volumes of the VI Cube. At the time, 2 x 300GB was recommended as being sufficient.
However, some have found benefits to putting different instruments on different drives as the library grew. They have also opted for other hard drive configurations to accommodate the extra data flow. Some of the solutions have been multiple eSATA drives or multiple firewire drives running from a PCIe host card. Some are using various RAIDs in addition to various FW and internal SATA options.
The more important issue here is bus speed and bus independence. More drives of a smaller size will help with seek times, but having additional data paths is the motivation here where having just two drives would compete for bus bandwidth. If the entire library were stored on two hard drives, the same data path clog potential would remain with multiple drives unless additional busses were created for that data.
How the library is split among hard drives is largely up to the way the user feels is best according to the needs of their projects. For example, some people write very busy string parts and have opted to put all of their violins and violas on one drive with their cellos and basses on another. Others prefer keeping all instruments in the same 'family' together.
At the end of the day, it all comes together inside your VSL Directory Manager and your DAW-- it all looks the same on the surface no matter how the library is split. As you work with your projects, you can always move various instruments to different hard drives (on the same machine) that are less active if it improves data flow. It will take some trial and error.
The one area where this gets tricky is when more than one computer is being used. For those who'd like to have all of their violins and violas on one machine, they are prevented from doing so simply because Solo Strings and Chamber Strings are each one complete bundle and cannot be split across two machines; different hard drives on one machine is doable. Different hard drives on different machines is not doable for a single volume/license.
My HDs:
MacPro: 10k 150GB SATA, 7200 300GB SATA
G5 2.5: 2 x 10k 150GB with eSATA II enclosure, internal 300GB SATA, 1.2TB eSATA II RAID (this drive contains other instruments that are never used with VSL concurrently)
As the VSL Team releases addition instruments, I will be adding an eSATA II host to the MacPro. Eventually, a third machine will be added with additional drives as well.
For data backup, all samples are archived onto several FW drives which are connected only when needed.