For open systems environments, including Windows, fixed-block architectures or fixed-block addressing is the most common basis for performing I/O operations to and from a disk drive. The most common I/O command protocol for open systems is the SCSI command set, not to be confused with parallel SCSI cabling, as there is a difference. The SCSI command set is implemented on different network and storage I/O interfaces or transports, such as Fibre Channel, iSCSI, SAS, InfiniBand and others.
Part of the SCSI command set protocol is an initiator (source) that requests an I/O operation (read, write or status inquiry) and a target (destination) device with a subaddress known as a LUN. The target can be a single disk in a JBOD array with each HDD having a different SCSI target and possibly a unique or selectable LUN of the device supports it. A common use of LUNs is in storage systems or arrays that incorporate some type of controller, usually with RAID where multiple HDDs are configured and aggregated into a RAID or volume group and then assigned a unique LUN and accessed via a SCSI target ID.
To learn more, check out chapters 3 (Networking with your Storage) and 4 (Storage and I/O Networks) in my book "Resilient Storage Networks" (Elsevier) along with other SearchStorage.com and Storage magazine tips and FAQs, including Tracking down those missing bytes.
About the author: Greg Schulz is founder and senior analyst with the IT infrastructure analyst and consulting firm StorageIO Group. Greg is also the author of Resilient Storage Networks (Elsevier) and a contributor to Storage magazine and other TechTarget venues.