There are two ways:
Research Disk Striping or RAID.
On your hard drive which can be accessed by going to "My Computer" and selecting the C: drive. If you have multiple hard drives then you will find more data in D:, E:, etc.
While external hard drives have the availability to work with multiple computers, it would be false to say every external hard drive works with every computer.
Yes, you can install multiple hard drives in your computer. If you have multiple internal bays then you can install drives inside the case. If not, you can buy external Firewire and USB drives that attach to the ports on the back of your computer. If you are unfamiliar with opening your computer, then choosing an external solution is the easiest and beth method.
Hard drive, Disk drive and FLoppy disk drive
C: drive is the main hard drive on your computer. Hard drives are identified by letter, with C: being reserved for the first bootable drive in a computer.
You have no hard drives listed. You have the ability to assign any drive letter (except the Main 'C' ) to your hard drives using "Computer Management"
You could get a USB external hard drive and plug it in
Commonly, external hard drives will acquire power from the computer it is connected to. It is possible, though, to connect external hard drives to an outlet and permanently to the computer, replacing the previous hard drive, though I highly do not recommend such.
Hard drives can make a big difference in computer performance. Consider a solid-state drive if you need quick drive access.
From Windows' nomenclature of hard drives connected to your PC... A and B are for 3.5" floppies, C is the main computer hard drive, and from D onward the letters are for additional drives.
The computer BIOS will decide which will be read first.
Assuming that when you say system unit of a computer you mean the computer case A.K.A. the tower, then no, hard drives are not always installed internally. Hard drives can be in external enclosures, hard drive docks, and even network attached devices.