It's a PowerBook, with one internal hard disk (one disk, one volume). I only have one single copy of OS X (bootable OS) on my machine. There is nothing else installed, I do not have two hard disks or volumes/partitions. It's all singles.
I did, however, install OS X from an external HD and had to boot off that in order to install, but I don't think it matters now because that external HD isn't plugged in and I am booting off the single volume, internal PowerBook HD, called Macintosh HD.
Now that is weird! 😱