PC data storage and hard drive guide

Data Storage Devices

Hard Drives

Hard drives are magnetic storage devices that contain several discs inside called "Platters" that are attached to a spindle motor. The number of platters varies depending on the capacity of the drive. Platters are coated with a film of magnetically sensitive substance that is primarily made of iron oxide. Another important ingredient is a thin layer of cobalt alloy. The read/write heads are responsible for reading and writing to the platters and are attached to the head actuator which is in charge of moving the heads around the platters.

The voice coil actuator is found in modern drives and assures that the heads are in proper position which ensures that the appropriate tracks are read. The guidance system used by the heads is called a servo. Its job is to position the head over the correct cylinder. The spindle motor is responsible for spinning the platters at a rate ranging from 3600 RPM to 10000 RPM depending on the drive. Heads typically have a coil of copper wire inside. Currents are passed through the wires which causes the surface underneath to become magnetized, creating 1 bit of data. The direction of the current passing through the wiring dictates the polarity of the magnetization, which creates a 0 or a 1. To read the data, the drive's electronics detect polarity differences.

The disk's surface has tracks that are rings that are located next to each other. Each platter has the same number of tracks, and the tracks on the outside are larger than the tracks on the inner part of the surface. A track location that cuts across all platters is called a cylinder. Each cylinder is divided into sectors that are 512K in size. The size of the sector determines the amount of data that can be written, and the amount that will be wasted if only a few characters are in a record. A one byte record written to a sector occupies the entire track in that sector.

Hard drive performance is measured as follows:

Access Time - This is a measure of the average time that it takes the drives R/W heads to access data on the drive.
Seek Time - This is the amount of time it takes for the drives head to move between cylinders and land on a particular track.
Data Transfer Rate - The megabytes per second(MBps) in which data is transferred between the drive and the system.

There are several different type of interfaces that can be used including IDE, EIDE and SCSI. Each IDE interface can support up to 2 devices. IDE devices of course each contain their own integrated controllers, and so in order to maintain order on the channel, it is necessary to have some way of differentiating between the two devices. This is assigning each device either a master slave designation using jumpers on the drive, and then having the controller address commands and data to either one or the other. Another option is to set the jumpers to cable select. This means that the position of the drive on the cable will determine its status. If you are using two drives on a single channel, it is important to ensure that they are jumpered correctly. Making both drives the master, or both the slave, will most likely cause problems.

Hard drives can be configured in a Redundant Array of Inexpensive Drives(RAID) configuration that is used for a variety of purposes including data recovery and increased read/write performance depending on the level of RAID employed. The RAID levels are as follows:

RAID Level 0
Disk striping will distribute data across 2-32 hard disks. This provides the fastest read/write performance as the system can access the data from more than one place. This level of RAID does not provide any redundancy.
RAID Level 1
Disk mirroring writes exact copies of data to more than one disk. Each disk or partition of a disk will contain the exact same data. If one hard disk fails, the data still exists on the other disk. This level of RAID also increases disk read performance as it can pull the data off of both disks.
RAID Level 2
Uses Hamming error correction codes, is intended for use with drives which do not have built-in error detection. All SCSI drives support built-in error detection, so this level is of little use when using SCSI drives. It is seldom used at all today since ECC is embedded in almost all modern disk drives.
RAID Level 3
Stripes data at a byte level across several drives, with parity stored on one drive. It is otherwise similar to level 4. It can be used in data intensive or single-user environments which access long sequential records to speed up data transfer. However, RAID-3 does not allow multiple I/O operations to be overlapped and requires synchronized-spindle drives in order to avoid performance degradation with short records.
RAID Level 4
Disk Striping in which the parity information is written to 1 drive at a block level. The parity information allows recovery from the failure of any single drive. The performance of a level 4 array is very good for reads(the same as level 0). Writes require that parity data be updated each time. The process offers no advantages over RAID-5 and does not support multiple simultaneous write operations.
RAID Level 5
Very similar to RAID level 4, however, parity information is written to each of the disks in the array. If one of the disks fails, the data can be reconstructed by installing a working hard disk. The parity information is used to reconstruct the data that was lost.

The following procedure outlines the installation of a hard disk.

Disconnect the power to the computer
Configure the appropriate master/slave settings or SCSI ID for the drive
Insert the drive into an available drive bay. If the drive is too small for the bay, you will need a mounting kit
Screw in the 4 screws - 2 on each side of the bay
If the drive is an IDE disk, connect the IDE cable to the drive. There should be a stripe along 1 edge of the cable. This stripe denotes pin 1. Pin 1 on the drive is usually closest to the power connector on the drive, however, you should consult the manufacturers documentation. Then connect the signal cable to the motherboard ID1 or ID2 interface making sure to note the pin 1 orientation there as well. If the drive is a SCSI drive, a SCSI cable would be connected from the drive to a SCSI controller card.
Connect one of the power supply's power connectors to the drive

Once the drive has been installed it must be configured for use in the following steps:

CMOS configuration - Newer BIOSes autodetection features will do this automatically. Otherwise, enter the setup utility during boot up and configure the drive.
Certain older drives types must be low-level formatted. Do not do this on IDE drives!
Partition the drive - Using the DOS utility FDISK, the drive can be partitioned into logical drives. The disk must contain an active primary partition that will be the C drive. An extended partition may also be created if desired. In Windows NT, the Disk Administrator program is used instead of FDISK. The size of the partitions can be set to a desired size, however, note the following:
- Windows 95 Rev A(FAT16) only supported partitions up to 2GB in size.
- Windows 95/98 OSR2(FAT32) supported drives up to 8GB.
- Even if the OS supports larger partition sizes, the BIOS must also support logical block addressing(LBA) or the maximum partition that you will be able to create will be either 504 or 528 MB.
Once the disk has been partitioned, it must then be formatted. This can be done using the DOS format utility.

Due to the magnetic nature of hard disks, they should remain clear of magnetic fields.

Floppy Drives
Floppy drives are also a form of magnetic storage that function similarly to hard drives. There is a spring loaded metal cover that is moved aside during operation that exposes a mylar disk that is coated with a ferro-magnetic substance. The drive's read/write heads access the disk as it turns on a spindle. Older PCs used 5.25 inch disks and drives that were able to hold 1.2mb of data. Modern 3.5 drives can hold 1.44mb of data. Given the popularity of newer storage types such as CDROM, ZIP disks and removable hard drives, it is not likely that further advancements to floppy technology will be made.

The following procedure outlines the installation of a floppy drive.

Disconnect the power to the computer
Insert the drive into an available floppy drive bay
Screw in the 2 screws
Plug the floppy cable into the drive and into the mainboard FD1 interface while noting the pin 1 orientation. Note the twist in the cable. Connecting the floppy to the last connector on the cable will make the drive an "A Drive" while plugging it in to the connector toward the middle of the cable will make it a "Drive B"
Connect one of the power supply's power connectors to the drive

CD-ROM
A beam is emitted by the laser and directed onto a single track on the disc by a prism/beamsplitter. As the disc rotates, the beam encounters a series of pits and landings that determine whether the beam is reflected back into the detector(from a landing) or scattered(from a pit). Light from the laser beam must penetrate a thin protective layer of plastic on the disc before striking the reflective coating that contains the pits and landings. As the disc rotates, light reflected from landings on the disk strikes the photo sensor producing a series of electrical pulses that are coordinated with a timing circuit to generate a stream of 1s and 0s that produce the binary code of information on the disc. The average storage capacity for a CD-ROM is 680mb of data.

Newer CD-ROM drives have the capability to record data. There are 2 main types of CD recorders.
CD-R (Recordable) - Uses a chemical layer with a thin metal layer(silver alloy or gold). “Burning” removes reflective parts to simulate pits and lands and represent 1s and 0s . CD-RW (ReWritable) - Uses phase-change material that crystallizes to write, and rewrite CDs through a heating and cooling process.

The following procedure outlines the installation of a CDROM drive.

Disconnect the power to the computer

Configure the appropriate master/slave settings or SCSI ID for the drive

Insert the drive into an available drive bay

Screw in the 4 screws - 2 on each side of the bay

If the drive is an IDE, connect the IDE cable to the drive. There should be a stripe along 1 edge of the cable. This stripe denotes pin 1. Pin 1 on the drive is usually closest to the power connector on the drive, however, you should consult the manufacturers documentation. Then connect the signal cable to the motherboard ID1 or ID2 interface making sure to note the pin 1 orientation there as well. If the drive is a SCSI drive, a SCSI cable would be connected from the drive to a SCSI controller card.

Connect one of the power supply's power connectors to the drive.

Tape Drives
Tape drives are another form of magnetic storage media that function similarly to the other forms of magnetic media. The tape is belt driven and read/write heads magnetize portions of the tape as it passes by them. Tape drives are typically used for backing up and storing data. Because they are comparatively slow, they are used to store data that does not need to be accessed very often. Older versions of tape drives were quarter-inch cartridges(QIC) that were approximately 6" x 4" in size. Improvements in encoding enabled advancements in the amount of data that could be stored on these tapes.

The newest advancements in tape technology have brought about Digital Audio Tape(DAT) and Digital Linear Tape(DLT). DAT tapes work in a similar fashion as a VCR tape and can store much larger amounts of data than the QIC formats. There are several different DAT standards as follows:

Standard	Compressed capacity	Uncompressed capacity
DDS-1	4 GB	2 GB
DDS-2	8 GB	4 GB
DDS-3	24 GB	12 GB

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