Hard Drives

– Part One

When we talk about computer memory we usually think of RAM – Random Access Memory. Computer memory is not just about RAM however. The hard disk is also a form of memory. When the computer is turned off all your work and other data can be stored on the hard drive for later retrieval.

The two main differences between RAM and the hard drive are the time needed to access data and the size of the storage area.  RAM is much faster than any kind of permanent storage system – access time is measured in nanoseconds. Hard drives get faster with every new wave of technology but they will never match the almost instantaneous access times that are possible with RAM.

The other main difference between RAM and hard drives is space. RAM is measured in Megabytes (MB) but hard drives are measured in Gigabytes (GB). One Gigabyte is equal to about 1000 MB.

As with just about every other computer component, the more hard drive space you have the better. There are some restrictions – some motherboards are limited in the size of drive they can access.  The same holds true for some operating systems.

How Hard Drives Work

Hard drives consist of several rotating platters inside the hard drive case.  Each platter is divided into tracks which are further divided into sectors.  Sectors are grouped into clusters and the size of the cluster is important for determining the maximum size of hard drive a particular operating system can use.

Data on the hard drive is accessed with a 'head' which moves from track to track and reads individual sectors.  The head is supported by an 'arm' which moves backwards and forwards to the different tracks on the platter.

The speed of the hard drive is determined by two things -- the rotational speed of the platters and the time needed to move the head to a new track.  There's also a rotational delay which is the time necessary for the desired sector to come around to the head.

Rotational speed is measured in RPM (Rotations Per Minute) and head movement is expressed as seek time.  Most hard drives are currently rated at 7200 rpm and the average seek time is about 9 ms.

Some drives have a rotational speed of 10,000 rpm.  The faster the rotational speed the faster the data can be accessed, although other factors such as the type of interface and the built-in cache play a part in transfer speed.  Transfer rates are part of the specifications of hard drives and you will see figures ranging from 200 MBps (megabits per second) to more than 500 MBps.

File Systems

Every hard drive needs to be formatted with a file system that is used for data access, navigation, and manipulation.  The file system is determined by the operating system and specifies how the data is stored and the maximum size of hard drive that can be used.

There are many different types of file systems.  The type of file system depends on the operating system of the computer. Microsoft Windows, Linux, and Macintosh all use different systems.

There are two types of file systems that can be used with Windows -- File Allocation Table (FAT) and New Technology File System (NTFS).  NTFS is available on Windows XP and has several features that make it more secure and reliable than FAT.

Prior to NTFS, Windows computers used a file system called FAT32.  This system uses 32-bit cluster numbers, which in theory should allow for hard drives up to 2 Terabytes (about 2,000 Gigabytes) in size.  FAT32, however, limits file sizes to a maximum of 4 GB and Microsoft limited FAT32 partitions to a maximum of 32 GB.

The 4 GB file size limit is a severe limitation for some computer applications like video editing.  Also, 32 GB hard drives are a bit on the small side for modern computers.  Most modern Windows-based computers use the NTFS file system which offers advantages in size and security.  There is no practical limit to either file sizes or hard drive size, and special features allow NTFS greater flexibility in file system management.

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