Why does the hard drive partitions always starts from C: drive ?

Microsoft assigns floppy drives as A: and B: drive. Hard drive partitions begin from C: drive.

As someone else stated, the Operating System reserves A: and B: for other media drives. However, on Linux, or other Unix based operating systems they just use the term File System!

Most commonly A: drive is used for floppy disk of size 3.5 inches, storage capacity is 1.44MB and B: drive is used for floppy disk of size 5.25 inches, storage capacity is 1.2MB. So, C: and remaining drive letters are created on Hard disk.

Floppy disks were invented before the hard disks came into existence. Earlier floppy disks were used to store even the Operating System.

This naming convention of "C: Drive" for Hard Disk partitioning is to avoid the Drive letter conflict when system is booted with DOS disk for recovering the system.

Sometimes, Floppy drives are used to update BIOS too. Even more, Windows XP asks for external driver source for SCSI Hard Disk. Some softwares, drivers and operating system files also use A: & B: drive letters as floppy drive references.

On IBM compatible computers running a Microsoft operating system such as Windows or MS-DOS, the hard disk drive is the C: drive because it is the first available drive letter for hard disk drives. The computer assigns the A: and B: drives to floppy disks and sometimes other removable media such as tape drives. As you install other hard disk drives, create new partitions, and add other drives such as CD-ROM or DVD drives, they will be assigned to other drive letters after C, such as D, E, F, G, etc.

For example, most computers today come with a floppy drive, hard disk drive, and disc drive, such as a CD-ROM, CD-R, or DVD drive. With this common configuration, your floppy disk drive would be assigned to the A: drive, your hard disk drive would be assigned to the C: drive, and your CD or DVD drive would be assigned to the D: drive. If no floppy drive comes with the computer because A: and B: drive letters are reserved for floppy drives your hard drive will still be assigned as the C: drive.