SCSI and it's definitions

Short explanation of SCSI

With SCSI there are *alot* of name games, and no hard standard.
  Sometimes you'll see "Ultra" in front, sometimes you won't.  Roughly
speaking, SCSI-1 is 40, SCSI-2 is 80, and SCSI-3 is 160.  As for pin
outs, you have 68pin, and 80pin.  80pin (also referred to as SCA) is a
hotswappable drive, almost always found in rackmountable servers, where
high availability is of concern.  68pin is what you have.  Old SCSI
stuff had 50pin interfaces as well.

Better Explanations

Here
http://www.seagate.com/support/kb/disc/scsirev.html

Hot-Swapping a Hard-Disk Drive

1. Open the lower plastic front door of the server.

2. If a padlock is installed on the metal door to the hot-docking bays, remove it.

3. Loosen the two captive spring-loaded thumbscrews securing the metal door to the chassis, and open the door.

4. Look at the banks of LEDs above the hot-docking bays to determine which drive is bad. Grasp the plastic lever (A), and pull it forward (1) to disengage the drive connector from the hot-docking backplane connector.

5. Remove the plastic carrier and drive assembly from the hot-docking bay (2), and place it on an antistatic surface.

6. Position the new plastic carrier and drive assembly so that it engages the hot-docking bay guide rails.

7. Gently push the drive into the bay until it docks with the hot-docking backplane connector.

8. Push the plastic lever to the right until it locks around the small metal posts, locking the drive in place.

9. Gently close the metal door, and secure it to the chassis with the two captive spring-loaded thumbscrews.

10. For security and to prevent unauthorized entry into the hot-docking bays, insert a padlock through the metal door and chassis and lock it.

11. Close the lower plastic front door of the server.

Hot Swapping a SCSI Hard-Disk Drive - LED Messages

A bank of three LEDs above each hot-docking bay monitors the drive's status. When the yellow LED on the right of the bank is on continuously, the user can hot swap (replace) a bad drive with a good one. The server does not need to be shut down to hot-swap a drive.

Drive Status LEDs

LED	Color	Status	Indicates
Left	Green	On continuously	Drive present and power on.
Middle	Green	Flashing	Drive activity.
Right	Yellow	On continuously	Drive fault the user can hot swap the drive at any time
			Powers down drive; it remains powered down until the user removes it from the bay and reinserts it or a new one, or power cycles the server.
			Fault stays on until RAID controller clears it.

Ordering SCSI Cables/Components

Specify the following

Bandwidth, Speed, Interface, and Interconnect
i.e.
Wide, Ultra SCSI, Single-Ended Interface, HD68