Hard Disk Drives

The hard disk plays a significant role in the following ways:

• Performance: The hard disk can directly effect the overall system performance. The speed at which a computer boots up and programs that are accessed is directly related to hard disk speed. This performance is critical when multitasking large amounts of data such as graphics, editing sound and video, or working with databases.

• Storage Capacity: A larger hard disk lets you store more programs and data. Newer software requires more space and faster hard disks to load them efficiently. Any computer enthusiast will tell you how a one time 20MB hard drive was huge. Ten years later, it will not even support the operating system.

• Reliability: A good quality hard disk, combined with regular maintenance and backups can ensure problem free usage.

Hard drive interfaces

The interface is the way the hard drive communicates with the rest of the system. In an effort to create larger, faster hard drives, several different standards have emerged. The following are the most common modern standards:

EIDE / ATA / Fast ATA-2 / Fast ATA-3

The EIDE has become a widely-accepted term in the industry. The original, base specification for the family of interfaces were called both IDE and ATA. The ATA-2 specification has been defined as an ANSI standard. The ATA-3 standard is a minor revision of ATA-2. A major reason for defining ATA-2 as a formal standard was to eliminate some of the incompatibility problems that plagued early IDE/ATA drives.

Most modern motherboards support the following features and transfer modes:

• Two EIDE controllers: Each with the capability of supporting two devices. This means most systems can support up to four EIDE hard drives, CD-ROM or backup devices. Each channel is shared by two devices that are configured as master and slave.

• PIO and DMA Modes: ATA-2 and ATA-3 includes support for modes 0, 1, 2, 3 and 4.

• Block Transfers: ATA-2 and ATA-3 adds commands to allow block transfers for improved performance.

• Logical Block Addressing (LBA): Requires BIOS support.

Ultra DMA / Ultra ATA / ATA-33 / ATA-66 / ATA-100 / ATA-133 / DMA-33

Ultra DMA, which also goes by several other names including Ultra ATA, ATA-33, and DMA-33 is not a formal standard but rather a term that refers to the use of the high speed transfer mode (multiword DMA mode 3), running at 33.3 MB/s or 66 MB/s. Special error detection and correction logic (CRC) is used to support the use of this high-speed mode. Ultra ATA maintains backward compatibility with the older standards.

Advanced Technology Attachment Packet Interface (ATAPI)

The IDE/ATA interface was originally designed to work only with hard disks. CD-ROMs used proprietary interfaces often integrated on the sound card. Backup drives often used the floppy disk controller which is very slow. It was obvious that there would be enormous performance advantages to using the standard EIDE interface to support devices other than hard disks. A special protocol was developed called the ATA Packet Interface or ATAPI. ATAPI CD-ROM, tape, cartridge drives use the same EIDE cable used by hard disks to be configured as master or slave just like the hard disk. Unlike the hard drive, however, a special ATAPI driver must be used to communicate with these devices. The driver is loaded into memory before the device can be accessed.

S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology)

S.M.A.R.T. is an interface between a computer's start-up program or BIOS and the computer hard drive. It is a feature of the Enhanced Integrated Drive Electronics (EIDE) technology that controls access to the hard drive. If S.M.A.R.T is enabled when a computer is set up, the BIOS can receive analytical information from the hard drive and determine whether to send the user a warning message about possible future failure of the hard drive.

Disk FAT Structures

The highest-level logical disk structures are the master boot record and partition tables, which define the way the entire disk is sized and organized.

Volume Boot Sector - Kept hidden, this sector contains information about what is executed when the disk is booted.  The Master Boot Record is the same for pretty much all Operating Systems.  It is located on the first Sector of the Hard Drive, at Cylinder 0, Head 0, Sector 1.  It is the first piece of code that your computer runs after it has checked all of your hardware (POST) and turned control of loading software over the hard drive.  It also contains the partition table, which defines the different sections of your hard drive.  Basically if anything happens to this little 512 byte section, your hard drive would be as good as formatted.

File Allocation Tables - The existing File Allocation Table (FAT) file system was invented in 1977 as a way to store data on floppy disks for Microsoft Stand-alone Disk Basic. Although originally intended for floppy disks, FAT has since been modified to be a fast, and flexible system for managing data on both removable and fixed media.

• FAT 16 is limited to 2GB per partition, drives that use it tend to have multiple partitions.  The first partition is the Primary Partition, and everything else is stored in the Extended Partition. While data is stored in 512-byte sectors on the hard disk, for performance reasons individual sectors are not normally allocated to files. The reason is that it would take a lot of time and space to keep track of pieces of files that were this small. The hard disk is instead broken into larger pieces called clusters, or alternatively, allocation units. Each cluster contains a number of sectors. Typically, clusters range in size from 2,048 bytes to 32,768 bytes, which corresponds to 4 to 64 sectors each. The file allocation table is where information about clusters is stored. Each cluster has an entry in the FAT that describes how it used. This is what tells the operating system which parts of the disk are currently used by files, and which are free for use. The FAT entries are used by the operating system to chain together clusters to form files. Damage to the FAT can of course result in data loss since this is where the record is kept of which parts of the disk contain which files. This is what scan disk checks for when booting when a computer was not properly shut down. 

• FAT32 was release in fall of 1996 and provided the following enhancements over previous implementations of the FAT file system:
  • Supports drives up to 2 Terabytes in size.
  • Uses space more efficiently. FAT 32 uses smaller clusters (e.g. 4kb clusters for drives up to 8GB in size), resulting in 10 to 15% more efficient use of disk space relative to large FAT drives.
  • More robust. FAT32 has the ability to relocate the root directory and use the backup copy of the FAT instead of the default copy. In addition, the boot record on FAT32 drives has been expanded to include a backup of critical data structures. This means that FAT32 drives are less susceptible to a single point of failure than existing FAT volumes.
  • More flexible. The root directory on a FAT32 drive is now an ordinary cluster chain, so it can be arbitrarily large and located anywhere on the drive. In addition, FAT mirroring can be disabled, allowing a copy of the FAT other than the first one to be active. These features allow for dynamic resizing of FAT32 partitions. Note, however, that while the FAT32 design allows for this capability, it will not be implemented by Microsoft in the initial release.

• NTFS stands for New Technology File System.  It was developed in the late 1980's and improved with the release of Windows 2000. Microsoft created NTFS to compensate for the features it felt FAT was lacking. These features include increased fault tolerance and enhanced security.

  • Fault Tolerance.  NTFS repairs hard disk errors automatically without displaying an error message. When Windows 2000 writes a file to an NTFS partition, it keeps a copy of the file in memory. It then checks the file to make sure it matches the copy stored in memory. If the copies don't match, Windows marks that section of the hard disk as bad and won't use it again (Cluster Remapping). Windows then uses the copy of the file stored in memory to rewrite the file to an alternate location on the hard disk. If the error occurred during a read, NTFS returns a read error to the calling program, and the data is lost.
  • Security. NTFS has many security options. You can grant various permissions to directories and to individual files. These permissions protect files and directories locally and remotely. NTFS also includes the Encrypting File System (EFS). EFS uses public key security to encrypt files on an NTFS volume, preventing unauthorized users from accessing those files. This feature comes in quite handy on a portable compute, for example. Lose a portable, and the files on its disk are fair game to anyone who knows how to get to them. EFS uses 128-bit (40-bit internationally) Data Encryption Standard (DES) encryption to encrypt individual files and folders. Encryption keys are implemented on a Windows 2000 domain or—in the case of a standalone computer—locally. The operating system generates a recovery key so administrators can recover encrypted data in the event that users lose their encryption key.
  • File Compression. Another advantage to NTFS is native support for file compression. The NTFS compression offers you the chance to compress individual files and folders of your choice.
  • Disk Quotas. Disk quotas allow administrators to manage the amount of disk space allotted to individual users, charging users only for the files they own. Windows 2000 enforces quotas on a per-user and per-volume basis.