Hard Drives

Keywords: blue laser, millipede, patterned media.

• Hard Drive Parts
• FATs

Hard Drive Parts

Here are the physical and logical parts of a hard drive.

• A hard drive consists of a number of platters on a spindle.
  • The platters are read and written to with heads for reading, writing, and aligning.
  • Each platter has two sides.
  • Each side is divided into a number of rings called tracks. The tracks are numbered 0 on the outside and usually go up to 1023 tracks.
  • All the tracks on the platter form a cylinder. Cylinders are also usually numbered 0-1023.
  • Each track is divided into sectors. Sectors are the smallest chunk of bytes usable on a hard drive. Sectors are usually 512 B but are always to the power of two.
  • Contiguous tracks form clusters.
• A hard disk has one MBR (Master Boot Record). A MBR holds the Partition Table which says how a disk is partitioned into up to 4 primary partitions, or 3 primary partitions and 1 extended partition.
  • A primary partition has a specific file system (eg FAT or NTFS) and may even have system file for a specific OS (eg W95 or WNT). A primary partition is assigned a logical hard drive letter.
  • An extended partition is a contiguous portion of the hard drive that is not occupied by a primary partition. An extended partition is not formatted but it can be divided into multiple logical drives, each with its own letter, and those logical drives can be formatted.
  • Space outside of these partitions are wasted and are not considered free space.
  • If a drive does not need a primary partition than the whole thing can be an extended partition.
  • Free space within an extended partition on one or more disks can be combined to form a volume set, i.e. a logical drive with its own letter.

A computer may have multiple hard drives. When additional hard drives are added to a machine, the assignment of logical drive letters follows particular rules.

• The 1st primary partition on the 1st drive always has drive C:.
• The 1st drive will get the next letter(s) if it has additional primary partition(s).
• The additional drive(s) will get the next letters before any logical drives in extended partitions on the 1st drive if they have primary partitions; other wise, any logical drives in extended partitions on the 1st drive get the next letters before any logical drives in extended partitions on the additional drive(s) get assigned a letter.

FATs

File Allocation Tables are used by the OS to point to and find all the parts of all the files on your hard drive. Here are three Microsoft FATs in historical order:

• FAT (Aka Virtual FAT or FAT 16).
  • Can be used by DOS, Win 3.x/9x/NT/2000 and OS/2.
  • Names follow the 8.3 naming convention. It must be an ASCII character except for spaces and the following: . " / \ [ ] : ; | = ,. All characters are effectively uppercase.
  • Works well with volumes less than 500 MB, becomes impractical when a volume reaches 1 GB, and has a maximum file and volume size of 2^32 B (4 GB) in NT and 2 GB in DOS.
  • Has 16 b cluster addressing and is thus limited to 2^16 or 65,535 clusters (entries) per volume.
  • Sectors are 512 B.
  • Logical drives should be limited to 2.1 GB.
  • Can enable share-level security on the directory level (access to any user via password).
• NTFS (NT File System).
  • Can be used by Win NT/2000.
  • Names can be up to 255 characters including extensions. It must be any Unicode character except for the following: ? " / \ < > * | :. Although it is only practical to use a  Windows ANSI character. Case is preserved but effectively irrelevant.
  • Works well with volumes above 400 MB and has a theoretical maximum file and volume size of 2^64 B (18 EB).
  • Has 64 b cluster addressing and is thus limited to 2^64 or roughly 1.8E19 clusters (entries) per volume.
  • Sectors can vary in size.
  • Allows permissions for individuals and groups to access individual files and folders.
  • Enables individual file and folder compression, as well as usage of compressed items without having to decompress.
• FAT 32
  • Can be used by Win 98/2000.
  • Works well up to 1 TB.
  • Has 32 b cluster addressing. 
  • Logical drives should be limited to 2 TB.

Here is a recommended layout for utilizing logical drives:

• C: for operating system and utilities.
• D: for applications.
• E: for data.
• F: for anything that needs a FAT drive.

This way if a failure occurs, it will usually occur on drives C: or D:, leaving data on drive E: intact. The FAT drive F: can be used by most operating systems.

For regular user, without multiple drives, RAID, etc. on big C: drive is just fine. Just make sure you back up.

2007-10-04 21:59:19Z