You can usually tell by looking at either the label on the drive or the connector to the controller.
Most SCSI drives are labeled such on the label that has the model and serial number on the drive. The connector to a SCSI drive is also different than an IDE drive.
SCSI-1 Narrow uses a 50-pin connector
SCSI-2 Wide uses a 68-pin connector
SCSI-3 wide uses a 68-pin connector
Ultra SCSI 160 uses an 80-pin connector
IDE drives use a 40-pin flat ribbon cable connected to the controller port that is usually on the motherboard. SCSI controllers in servers are usually connected via a PCI slot in the server. The cables go from the controller in the PCI slot to the disks.
Most PCs ship with IDE drives. Most servers ship with SCSI drives. The difference between the two drive types is how you access the data on the drive. IDE or "integrated drive electronics" are "smart" drives as most of the electronics that contain logic are located on the drives themselves. SCSI drives are "dumb" drives and the controller contains all the smarts.
The main difference is performance and high availability. SCSI controllers offload the processing of disk I/O from the servers CPU so it can be off doing other more important things. IDE drives usually use the CPU in the PC to do most of the processing for data I/O. SCSI controllers can also group disks into "RAID sets". This means the controller can group multiple independent disks into a RAID group, which makes all the independent disks look like one big disk to the server.
RAID controllers can also create "parity" information for every write to the raid group. This parity information is used to recreate data if a disk drive in the RAID group fails. This provides data protection and continuous data access in the event of disk hardware failures. Creating RAID sets can make data I/O go much faster in many cases too. That's why high-end servers use SCSI and cheap PC's usually use IDE.
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This was first published in August 2002