The first popular serial interfaces for hard disks were SSA (Serial Storage Architecture, developed by IBM) and Fibre Channel in the variant (FC-AL Fibre Channel arbitrated loop). SSA disks are now virtually no longer manufactured, but Fibre Channel disks are still built for use in large memory systems. Fibre Channel designates the protocol used, not the transmission medium. Therefore, these disks have despite their name, no optical characteristics, but an electrical interface.
External Hard Drives
External hard drives are available as physical storage (block) devices, USB, Firewire, eSATA disks or logical mass storage (NAS) with connection via Ethernet and / or Wi-Fi. In the former case the hard drive is from the perspective of a particular operating system drive, in the second case the entire NAS is a special tailed computer over a network.
For the connection of external hard drives universal interfaces such as FireWire or USB, eSATA are often used. This is between the disk and the interface to a bridge that has a USB, Firewire, eSATA port or multiple ports on one side of a PATA or SATA connector and on the other side.
In such external devices two hard drives are partially installed in a housing, but appear to the outside as a single disk (RAID system). Similar to NAS systems where a network connection is then available. Here the expansion capacity is virtually unlimited.