Answer:
The oldest of the hard drive connections are the SCSI (Small Computer System Interface) types. The 50 pin flat ribbon cable connectors were found in all sorts of computers and servers from their introduction in 1986 until the mid 1990's.
They are still in use today, albeit in their updated Ultra SCSI form which uses either 68 or 80 pin connectors along with a pinless plug that is similar to a USB connection. Up to 16 drives/devices can be connected through SCSI connections. {The common alternative to SCSI connectors is the IDE connection. Also known as ATA or PATA, IDE drives use a 40 pin flat ribbon cable and are limited to just two drives per cable. These drives were initially developed for IBM PCs.
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Introduced in 2005 as a replacement for Ultra SCSI, SAS drives, or Serial Attached SCSI, share many common features with regular SCSI but offer many improvements. While SCSI drives can only run a single communication in the order it's received, SAS drives allow for multiple simultaneous communications through less numbers of pins and higher speeds. SAS plugs look similar to those found on SATA devices but have seven additional pins for a total of 29 (14 for data, 15 for power). SAS systems can also handle upwards of 66,000 devices per controller.
SATA, or Serial ATA, drives are similar to SAS but have a simplified plug with 15 power and seven data connections. SATA drives can technically plug directly into any cable that accepts a SAS drive but it may not been by the computer it is connected too if it doesn't support SATA drives on a SAS system.
Finally, FC or Fibre Channel drives are designed for larger storage systems that use fiber optic interconnects between the server and drive enclosures. The connections on Fibre Channel drives typically prevent them from being used with other drive types due to their specialty nature but since they are based on SCSI technology, they work easily with many different server systems.
