It contains everything from the operating system, application software, utilities, and files for on going work. There are various types depending on your needs. I like to continue including very old ones. Some are still in use for very basic applications. It is interesting to see the evolution of the personal computer.
All drives come as masters or primary drives.
You can add a secondary or slave provided it is the same type. ST-225 is made by Seagate and has 10 to 20 MB and is found in XT types. ST-506 has 20 to 40 MB and can be in PC's built before 1987. The later is fastest, but not fast enough to meet the demand. Starting at 40MB up to 850MB of space.