Personally I just use partitions. I suppose there are advantages of using separate physical disks but I can think of one disadvantage that is more important to me...MONEY! Anyways, as for why your book may have said that separate disks are better....
The first thing that comes to mind is a disk failure. If your partitioned hard drive fails, all of your operating systems are done and your PC is unusable. If you would have had separate disks, you could still use your other ones!!
The next thing that comes to mind is read/write speeds. Hard drives start from the outside of the platter and write inwards. The outside of the disk rotates faster than the inside, and therefore data can be read faster from the outside than the inside. As a result, your first partition/operating system will run fast, and then each subsequent one will run slower (marginally, I suppose). Having separate disks would eliminate this problem
Hope this helps[/QUOTE
Yes, I get it now. If one OS on a partitioned single HD crashes, it takes the other OS down with it. So is it true that to get full size tower with dual-HD, my only option is to special order from factory, or custom-build from a computer shop? Been looking for dual-HD models on google without success, and someone said dual-HD is not available "out-of-the-box".