Hot swap and hot plug are basically the same thing. It means the device, usually a hard drive, can be taken out and connected without powering down the system. The ability to hot swap drives is very important in servers where powering down to fix a hardware issue is not possible. SCSI drives are usually associated with the ability to hot swap. Many come with the option for a 80pin SCA connector with the data cable and power connection integrated into an industry standard adapter. 68pin SCSI and IDE drives can also be hotswapped if placed in a tray or drive bay that supports it.
As for hot spare, it offers an extra level of security. An extra drive is kept running in a system in anticipation of a failure. As soon as a drive craps out, the data from the lost drive will rebuilt on the hot spare. Example, you have 5 x 9GB drives. They are in RAID5 configuration with one hot spare. That means 27GB is available for use. 4 drives in the RAID5 array (N-1 = Available Capacity) and the 5th drive is the hot spare.
Believe it or not, drives are not the only things that can be hotswapped. CPUs and PCI cards can also do the same on the right system. I remember the first time I was told by HP tech support to, "Just reach in there and yank out the bad CPU." I had to double check to make sure I didn't need to power down the server.
Example of a hot swap/hot plug drive tray.
Hope this helps.
Windogg
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