Hard disk drives are accessed over one of a number of bus types, including parallel ATA, Serial ATA, SCSI, Serial Attached SCSI, and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses that they cannot communicate with natively, such as IEEE 1394 and USB.
Back in the days of the ST-506 interface, the data encoding scheme was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5 megabits per second. Later on, controllers using 2,7 RLL encoding increased disk capacity by fifty percent.
Many ST-506 interface disk drives were only specified by the manufacturer to run at the lower MFM data rate, which other models were specified to run at the higher RLL data rate. In some cases, a disk drive had sufficient margin to allow the MFM specified model to run at the faster RLL data rate; however, this was often unreliable and was not recommended.
Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time however, 15 or 20 megabit ESDI disk drives weren’t downward compatible. ESDI disk drives typically also had jumpers to set the number of sectors per track and sector size.
SCSI originally had just one speed, 5 MHz (for a maximum data rate of five megabytes per second), but later this was increased dramatically. The SCSI bus speed had no bearing on the disk’s internal speed because of buffering between the SCSI bus and the disk drive’s internal data bus; however many early disk drives had very small buffers, and thus had to be reformatted to a different interleave when used on slow computers.
ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and couldn’t run in a master/slave setup (two disks on the same cable).
Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.
FireWire/IEEE 1394 and USB (1.0/2.0) HDDs are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/ IEEE 1394 models are able to daisy-chain in order to continue adding peripherals without requiring additional ports on the computer itself.