No, C & D might simply be two partitions of one physical drive.
Each disk partition, regardless of whether there are more than one physical drives in the system, is given a drive letter.
The answer depends on what level you are talking about and what kind of computer you are talking about. A standard PC running a Microsoft Windows OS is going to identify the physical drives by numbers, starting with "drive 0". The physical drives can then be broken into virtual drives and assigned drive letters. You will usually be using letters to work with the drives. Bytes are the units that measure the capacity of the drive, so the bytes are not really used to identify the drive.
I drive to work every morning. She drives a car. My computer has one drive. / My computer has two drives. My brother drives me crazy. His thirst for knowledge drives him to study harder.
MSDOS and Microsoft Windows identifies drives by single letters (many other operating system allow drives to be named with words or phrases):drive A the first internal floppy diskette drive (usually not used anymore)drive B the second internal floppy diskette drive (usually not used anymore)drive C the first internal hard disk drivedrives D through Z additional drives, may be internal or external, hard disk drives or optical drives, etc.
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.
Go to Start - Control Panel - My Computer. It should show you the hard drives on your system.
Servers can have physical drives just like your computer or they can have network attached drives, which is really another server that has many drive that are provisioned to provide more space than a regular disks can. A server, unlike your computer, must be able to store much more data. With modern servers, the server's hard drive is only used to load the operating system. The connected drives offer storage only.
Usually the network drives start at the end of the alpha and go to the start, so Z drive on most networked computers is a physical drive somewhere on the network
Usually the network drives start at the end of the alpha and go to the start, so Z drive on most networked computers is a physical drive somewhere on the network
Atmost four drives can be mapped to a windows 2000 operating system computer. It can have different names like z name can be used for a drive.
USB
Hard drive, Disk drive and FLoppy disk drive