Using Hard Disks

All disk drives must be initialised before they can be used to store information. This process organises the areas of the disk in a way that can be understood by the computer. Most modern machines come with the internal drive already prepared, allowing you to start work immediately. However, if something really nasty happens you may need to re-initialise the disk.

Some varieties of disk formatting application split the job up into more than one operation. Where the disk is new or damaged the process must begin with low-level formatting, which is then followed by high-level formatting. Unfortunately, some applications refer to low-level formatting as formatting, whilst high-level formatting is known as initialisation.

The three operations performed by a formatting application are:-

Low-Level Formatting

This allows the drive work with a computer at its most basic level. The formatting process organises the usable sectors in each track or cylinder of the disk into packages known as allocation blocks, each of which is given an address marker. The largest possible block can occupy an entire cylinder, whilst the smallest possible block is the same size as a single sector.

Usually, one or more alternate or spare sectors are assigned to replace any faulty sectors that are found in each track. Details about these bad data blocks are stored in a special area of RAM assigned to the drive, allowing the computer to use the spare sectors instead of faulty blocks.

High-Level Formatting

This process, also known as logical formatting, rebuilds the disk directory, forming a logical disk structure that can be understood by the operating system. It creates the following items:-

  1. Boot Blocks, marking the start of a disk, memory setup and startup files
  2. Volume Information Block, setting a folder for the System and Finder
  3. Volume Bit Map, allocating where data is stored on the disk

In Mac OS X, this is the same as erasing a disk and therefore doesn’t touch the low-level formatting unless a problem is encountered. The process can also remove and replace soft partitions and drivers (see below) prior to erasing.

Updating the Driver

Most formatting applications let you to manually update the driver software that’s hidden away on your hard disk. In most instances a high-level format will also install it. Ideally, you should update your driver whenever you upgrade your system, although this can often be impractical if you have a drive that’s incompatible with Apple’s Disk Utility application. Even so, it’s a good idea to contact the vendor of a non-standard drive to see if there’s an update available.

  Using a Formatting Application

If you have an Apple drive or one of many third-party drives, you should be able to initialise it by starting up from your Mac OS X Installer CD and running Disk Utility from the CD. Simply click on the Erase tab in the application, select the required formatting in Volume Format and click on Erase. The following points are important:-

  Formatting Options

The Disk Utility application can format discs using Apple’s Mac OS Extended format, also known as Hierarchical Filing System Plus (HFS+), or the Unix Filing System (UFS). However, Mac OS Extended formatting is usually preferred, unless you really have to employ UFS, perhaps for interchanging drive mechanisms with users of non-Apple Unix machines.

When formatting a disk you should set Volume Format in the Erase pane of Disk Utility to Mac OS Extended (Journaled). This allows the computer to keep a log of all changes to the disk, enabling a fast and effective repair to the drive following a crash. If it isn’t already enabled, simply run Disk Utility, select the disk and choose File ➡ Enable Journaling.

  Windows Compatibility

Mac OS X 10.3 and higher versions of the system can also mount drives that are formatted using the usual Windows mechanism, known as New Technology Filing System (NTFS). Assuming your machine has suitable connections for the drive concerned (or an appropriate interface card), you can plug such a drive into your machine and read the data off the disk, although you can’t write anything to it. In addition, when copying files you may encounter problems with files that have long names or illegal characters.This trick also works to a lesser extent in Mac OS X 10.2.

Special Options

The most common options provided by specialised formatting applications include:-

Blind Writes

This feature, which is only provided in advanced formatting applications, prevents the computer from checking data as it’s recorded onto the drive. Although this can double the writing speed, there’s a serious risk to data integrity. For this reason, you should leave this option turned off!

Name

In most applications you can enter a name for the drive, such as Macintosh HD or Hard Disk. When reformatting a drive for existing software it’s advisable to keep the drive’s original name, otherwise some applications may lose track of the disk and the files that are on it.

Icon

Most non-Apple applications let you choose an icon before formatting. If those provided aren’t to your taste just use any icon. Once formatting is complete, highlight the disk in the Finder, choose File ➡ Get Info and then paste a new icon of your own choice in place of the original. If you have a drive with partitions (see below) you can paste a different icon onto each one.

Disk Drivers

Before upgrading the system on an Apple-compatible drive you should update its driver using the latest version of Disk Utility, as supplied with your new version of Mac OS X. If you have a non-Apple drive you should check with your supplier to see if an updated driver is available.

Assuming the driver is working correctly, the drive should appear on the desktop, meaning that the disk is mounted. When a removable disk is inserted it should mount automatically, a process known as auto-mount. A drive inserted before startup usually mounts automatically, since the system reads the driver from it during the startup process.

Partitions

Partitioning splits a drive into separate compartments known as logical volumes. At least one of these, usually the largest, is used by Mac OS X for any files created within the Mac’s environment. Others can be used for alternative filing and operating systems, disk driver(s) and optional free space.

The advantages of introducing extra partitions are debatable. For the average user who only employs Mac OS X there aren’t any real gains. However, if you have software that emulates other systems, such as Virtual PC (Connectix), it may be useful to set up an entirely separate partition, thereby fully segregating the data that’s used for the different platforms.

Two types of partitioning are used:-

Hard Partitions

Hard partitions are created during low-level formatting by dividing the disk into groups of tracks.

Soft Partitions

Soft partitions, which can be created using some formatting applications, allow you choose your partitions after initialisation. Although the size of partitions can be altered at any time you should back up your data before making any adjustments.

You can use the Partition tab in Disk Utility create partitions, although they can’t be introduced to the startup drive.

Verification

Drive verification checks the disk’s reliability. The process is often undertaken automatically by a formatting application after initialisation a disk or may be implemented manually.

Two methods are used:-

Full Verification

This test, which is almost completely foolproof, records onto the entire surface of the disk, destroying all the data, identifying any faulty sectors in the process. A record is kept of these sectors, allowing them to be avoided in all subsequent operations.

CRC Verification

A cyclic redundancy check (CRC) is a non-destructive process in which the data in each sector is compared with related redundant values located at the start of each sector. The process doesn’t normally harm your data but may identify faulty sectors, making the information inaccessible.

The most common system is known as CRC-16, in which data is split into blocks of 128 bits (16 bytes). A single bit is then chosen from each byte, in a defined sequence, to create the CRC block itself. These redundant bits are then used to check the validity of the data. Unfortunately, the technique isn’t foolproof, since two wrongs can often make a right!

  Drive Solutions

Optimisation

After a period of use, a disk can become fragmented. Parts of each file get scattered across the disk and are interspersed with other material, slowing down the drive as it hunts for all the pieces. This problem can be solved by optimising or defragmenting the drive using a special application.

The optimisation process extracts files from the disk and writes them back into contiguous sectors on the same drive. Each file then occupies less disk area, thereby reducing wear and tear on the mechanism. However, a small amount of disk capacity can be lost in the process.

Other Hints

References

MacWorld magazine (UK), IDG Communications, 2004

Panther Meets NTFS, Sylvester Roque, ATPM 1002

©Ray White 2004.