The first IBM 5150 PCs had two 5.25-inch 160 KiB single sided double density (SSDD) floppy disk drives.
As two heads drives became available in the spring of 1982, later IBM PC and compatible computers could read 320 KiB double sided double density (DSDD) disks with software support of MS-DOS 1.25 and higher.
The same type of physical diskette media could be used for both drives but a disk formatted for double-sided use could not be read on a single-sided drive.
PC DOS 2.0 added support for 180 KiB and 360 KiB SSDD and DSDD floppy disks, using the same physical media again.

The disks were Modified Frequency Modulation (MFM) coded in 512-byte sectors, and were soft-sectored.
They contained 40 tracks per side at the 48 track per inch (TPI) density, and initially were formatted to contain eight sectors per track.
This meant that SSDD disks initially had a formatted capacity of 160 kB, while DSDD disks had a capacity of 320 kB. However, the PC DOS 2.0 and later operating systems allowed formatting the disks with nine sectors per track.
This yielded a formatted capacity of 180 kB with SSDD disks/drives, and 360 kB with DSDD disks/drives.
The unformatted capacity of the floppy disks was advertised as "250KB" for SSDD and "500KB" for DSDD ("KB" ambiguously referring to either 1000 or 1024 bytes; essentially the same for rounded-off values), however these "raw" 250/500 kB were not the same thing as the usable formatted capacity; under DOS, the maximum capacity for SSDD and DSDD disks was 180 kB and 360 kB, respectively.
Regardless of type, the file system of all floppy disks (under DOS) was FAT12.

After the upgraded 64k-256k motherboard PCs arrived during early 1983, single-sided drives and the cassette model were discontinued.

IBM's original floppy disk controller card also included an external 37-pin D-shell connector.
This allowed users to connect additional external floppy drives by third party vendors, but IBM did not offer their own external floppies until 1986.

The industry-standard way of setting floppy drive numbers was via setting jumper switches on the drive unit, however IBM chose to use instead a method known as the "cable twist" which had a floppy data cable with a bend in the middle of it that served as a switch for the drive motor control.
This eliminated the need for users to adjust jumpers while installing a floppy drive.
The first IBM 5150 PCs had two 5.25-inch 160 KiB single sided double density (SSDD) floppy disk drives. As two heads drives became available in the spring of 1982, later IBM PC and compatible computers could read 320 KiB double sided double density (DSDD) disks with software support of MS-DOS 1.25 and higher. The same type of physical diskette media could be used for both drives but a disk formatted for double-sided use could not be read on a single-sided drive. PC DOS 2.0 added support for 180 KiB and 360 KiB SSDD and DSDD floppy disks, using the same physical media again.
The disks were Modified Frequency Modulation (MFM) coded in 512-byte sectors, and were soft-sectored. They contained 40 tracks per side at the 48 track per inch (TPI) density, and initially were formatted to contain eight sectors per track. This meant that SSDD disks initially had a formatted capacity of 160 kB, while DSDD disks had a capacity of 320 kB. However, the PC DOS 2.0 and later operating systems allowed formatting the disks with nine sectors per track. This yielded a formatted capacity of 180 kB with SSDD disks/drives, and 360 kB with DSDD disks/drives. The unformatted capacity of the floppy disks was advertised as "250KB" for SSDD and "500KB" for DSDD ("KB" ambiguously referring to either 1000 or 1024 bytes; essentially the same for rounded-off values), however these "raw" 250/500 kB were not the same thing as the usable formatted capacity; under DOS, the maximum capacity for SSDD and DSDD disks was 180 kB and 360 kB, respectively. Regardless of type, the file system of all floppy disks (under DOS) was FAT12.
After the upgraded 64k-256k motherboard PCs arrived during early 1983, single-sided drives and the cassette model were discontinued.
IBM's original floppy disk controller card also included an external 37-pin D-shell connector. This allowed users to connect additional external floppy drives by third party vendors, but IBM did not offer their own external floppies until 1986.
The industry-standard way of setting floppy drive numbers was via setting jumper switches on the drive unit, however IBM chose to use instead a method known as the "cable twist" which had a floppy data cable with a bend in the middle of it that served as a switch for the drive motor control. This eliminated the need for users to adjust jumpers while installing a floppy drive.
IBM Personal Computer