The 5150 could not itself power hard drives without retrofitting a stronger power supply, but IBM later offered the 5161 Expansion Unit, which not only provided more expansion slots, but also included a 10 MB (later 20 MB) hard drive powered by the 5161's own separate 130-watt power supply.
The IBM 5161 Expansion Unit was released during early 1983.

During the first year of the IBM PC, it was commonplace for users to install third-party Winchester hard disks which generally connected to the floppy controller and required a patched version of PC DOS which treated them as a giant floppy disk (there was no subdirectory support).

IBM began offering hard disks with the XT, however the original PC was never sold with them.
Nonetheless, many users installed hard disks and upgraded power supplies in them.

After floppy disks became obsolete during the early 2000s, the letters A and B became unused.
But for 25 years, virtually all DOS-based PC software assumed the program installation drive was C, so the primary HDD continues to be "the C drive" even today.

Other operating system families (e.g. Unix) are not bound to these designations.
The 5150 could not itself power hard drives without retrofitting a stronger power supply, but IBM later offered the 5161 Expansion Unit, which not only provided more expansion slots, but also included a 10 MB (later 20 MB) hard drive powered by the 5161's own separate 130-watt power supply. The IBM 5161 Expansion Unit was released during early 1983.
During the first year of the IBM PC, it was commonplace for users to install third-party Winchester hard disks which generally connected to the floppy controller and required a patched version of PC DOS which treated them as a giant floppy disk (there was no subdirectory support).
IBM began offering hard disks with the XT, however the original PC was never sold with them. Nonetheless, many users installed hard disks and upgraded power supplies in them.
After floppy disks became obsolete during the early 2000s, the letters A and B became unused. But for 25 years, virtually all DOS-based PC software assumed the program installation drive was C, so the primary HDD continues to be "the C drive" even today.
Other operating system families (e.g. Unix) are not bound to these designations.
IBM Personal Computer