Linux+: Hardware Part 13 – Hard Disk Basics Hard Disk Drives (HDD) are used as storage media. The data stored on the HDD can be read and written randomly or sequentially. Data can be stored long term since electricity is not required to maintain the stored data. NOTE: Data can be placed on the drive sequentially, which is preferred, but data tends to become fragmented. Fragmentation occurs when data is not stored sequentially but randomly. Hard Disk Drives are characterized by size and performance. Size was originally in the Megabytes (MB), but reached the Gigabytes (GB), and now, has reached the Terabyte (TB) range. Performance is rated by speed of the various parts of the HDD. Each HDD is made up of one or more platters. Each platter is coated with a magnetized material (ferromagnetic material) which in most cases makes the platter appear as a mirror. The platter can spin at a speed over 15,000 Revolutions per Minute (RPM). The platter can store data on both sides by using a read/write head on an arm. The arm moves at a set speed to a location from the inner portion of the platter to the outer portion. Once the required data is under the read/write head, the magnetized portion can be read as being magnetized (on - 1) or not magnetized (off - 0). The three main parts of performance are as follows: 1. Access Time – Time to move heads to location 2. Latency – Time to move the file around to the head’s location 3. Rotation Speed – Platter speed in Revolutions Per Minute (RPM) NOTE: Ferromagnetism is a type of magnetism which is so strong, it can actually be felt. Because of the strength, the magnetic detector (read head) does not have to be strong which helps reduce the cost of HDD. Hard Disks are usually internal devices, meaning they are inside the computer case and referred to as Fixed Disks. External Hard Disks do exist and are becoming more popular since the devices can be connected to any system, by Universal Serial Bus (USB) or FireWire. External drives allow for data to be portable and more easily shared. Data is transferred from the HDD to the system by a cable connecting the HDD to a Controller. The Controller can be an Expansion Card or built onto the motherboard. The cable varies in size depending on the type of HDD (IDE, EIDE, SCSI, etc.). NOTE: The types of HDD will be covered in future articles. The data cable has a mark along one side, usually red (as shown in Picture 1) or blue, but can be any color. PICTURE 1 The side with the marking denotes that that side is Pin 1. The connector is placed into the Hard Drive where Pin 1 is next to the power connector as shown as Picture 2. PICTURE 2 NOTE: The left side of Picture 2 are the connections for the cable. The far right side is for the power cable. Between the two are the jumpers for configuring the drive within the system (covered within the individual articles for IDE and SCSI). The pictured HDD is an IDE drive. The connector on the controller also shows which pin is Pin 1 (Picture 3). Some may designate Pin 1 with a triangular arrow as shown in Picture 4. PICTURE 3 PICTURE 4 HDD contain cache to allow data being sent to or from (usually to) the drive to be stored while being written. Without the cache the system would have to wait to write data to the drive. The cache can hold the information and continually add new data as the cached data is written. If the system bus is busy the data being read from the disk can be held in cache until the data can be transferred across the system bus. The read/write heads float on a thin layer of air above the HDD platter/platters. Imperfections on the platter can cause the heads to make contact with the platter. If contact occurs, portions of the magnetic media can be scratched or removed which can cause data loss. Dust inside the HDD can also come between the heads and the platter causing the same problem. Power loss or fluctuations can also cause the heads to ‘drop’ when the spinning platter stops and air stops moving within the HDD case. NOTE: Take care to include an Uninterruptable Power Supply (UPS) to prevent power outages and fluctuations.