MS-DOS generally has two distinct types of disk drives: floppy drives and hard drives. Floppy drives are fixed size drives generally with capacities of 360kB, 720kB, 1200kB, 1440 kB or 2880kB (though there are older 160kB, 180kB, and 320kB models). Hard drives can come in different sizes with early models generally 10MB and more.
In a modern context, disk image files capture the files from a physical disk and store it as a single, specially formatted file called a disk image. The disk image may be used with modern software (or hardware) for various purposes:
- Archiving physical disks
- Taking archival snapshots and making physical media copies
- Perform forensic studies of physical disks
- Transfer files from one environment to another
- Emulating vintage systems
Disk images may now be created that are not copies of physical disks but rather containers to put vintage software for use in vintage hardware or emulators. While appearing to be a clunky way of aggregating files (compared to zip files, etc.), the disk image is useful in a number of scenarios.
Disk Basics
How disks are physically structured and how data is recorded on them is a lengthy discussion. See Brutman Working with Disks for some background.
With physical disk drives being circular, areas were broken into circular tracks. Pieces of the circle, semicircular, were usually of a fixed size and called sectors. (Wikipedia) In general, each sector of an IBM compatible disk has 512 byte sectors. The number of sectors per circular track varies depending on the disk density.
Files contain one or more sectors. One or more sectors can be aggregated into a cluster for ease of allocation. Special data sections on the disk keep track of which locations on disk belong to what files.
MS-DOS assigns each disk a drive letter to identify different drives, followed by a colon. Floppy drives were generally designated A: and B:, hard drives usually started at C:. There are exceptions, of course. Also software can be used to create special drives that don't correspond to physical local disks but are software defined. Examples may be a network drive or RAM disk.
Hard drives used to sometimes be bigger than the maximum drive letter size. They can be divided into smaller virtual hard drives through a process called partitioning. A 120MB hard drive using the older FAT16 disk format can only handle 32MB drives. Partitioning it to use 4 partitions of 30MB would use the entire disk. Each partition generally has a drive letter (C:, D:, etc.)
