You could add a hard disk to the original PC, but it wouldn't boot from it as the code in the BIOS wouldn't recognize your HD controller card (initially MFM like you said, a few years later RLL before we moved to IDE/ATA and eliminated such complications).

With the XT the BIOS would search upper memory for extra ROMs and the one in your HD controller card could patch the entry code for INT 13h so it could try to boot from the HD. If that failed it called the original INT 13h which would try to boot from a floppy.

On the software side we had a similar evolution. The first versions of DOS included all the drivers for all devices you might have. If some manufacturer wanted to add some new hardware it would have to ask Microsoft to compile a new version of DOS just for their machine. That didn't scale at all and soon the extra drivers were split into their own .sys binaries which could be loaded by autoexec.bat as needed.