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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Logical Structures and File Systems | Disk Partitioning and Formatting Programs ]


Every operating system comes with an appropriate utility for partitioning hard disks. The program used on most PCs is the one supplied with DOS and consumer versions of Windows (Windows 9x/ME). It is called FDISK, which stands for "fixed disk", an older term for hard disk. FDISK is used only for partitioning FAT family file systems (FAT12/FAT16/VFAT/FAT32), and allows you to perform the following functions:

  • Create Partitions: FDISK allows you to create a primary partition or logical volumes. To create a logical volume you must of course first create an extended DOS partition, since the logicals are contained within the extended partition.
  • Set Active Partition: You can use FDISK to set the primary partition on your boot disk active, so that it can boot. It's quite silly that FDISK doesn't do this automatically when you create the boot primary partition (since there can only be one enabled primary DOS partition anyway), but in fact you must do this manually in many cases. (At least FDISK warns you when no disk is set active, via a message at the bottom of the screen.)
  • Delete Partitions: FDISK will let you delete partitions as well. This is the only way to change the size of a partition in FDISK: delete the old one and create a new one with the new size. If you want to change the size of the primary DOS partition using FDISK you must delete every FAT partition on the disk and start over... This is one reason why third-party partitioning programs have been so successful.
  • Display Partition Information: The last basic option that FDISK gives is to display the partition information for the system. It will first show the primary and extended partitions and then ask you if you want to see the logical drives within the extended partition. In fact, if you want to see this information, you can just do "FDISK /STATUS" from a DOS command line or Windows DOS box. This will show you the partition information without actually taking you into FDISK, and therefore, you run no risk of accidentally doing something you'll wish you hadn't.

Some important points that you should keep in mind when using FDISK:

  • Be Careful: With just a few keystrokes, FDISK can wipe out part or all of your hard disk. Generally speaking, don't use FDISK unless you need to, and make sure you understand what you are doing before you begin.
  • Run It From DOS: Windows 9x allows you to run FDISK direct from the graphical user interface, and even use it while other applications are open and running. Since FDISK alters critical disk structures at a very low level, running it while files are open and other applications are using the disk is asking for trouble. To be safe, always exit to DOS ("Restart the computer in MS-DOS mode") before using FDISK (except for using "FDISK /STATUS", will work safely from within a DOS box, as mentioned above).
  • FAT32 Support: The version of FDISK that comes with newer versions of Windows supports the creation of partitions that use the FAT32 enhanced file system for larger volumes. Some genius at Microsoft, however, decided not to call it FAT32 within this program. Instead, when you run FDISK on a system that has FAT32 support, and a hard disk over 512 MB (the minimum for using FAT32), you will receive a message asking you if you want to "enable large disk support". If you answer "Y" then any new partitions created in that session will be FAT32 partitions. If you accidentally hit "N" or don't understand the question, FAT32 will be disabled (which routinely causes confusion on the part of many newer PC users...)

Tip:  It is often useful to include FDISK as one of the programs on a bootable floppy. This way you can use it when setting up new hard disks.

Introductory page to the Windows 9x FDISK program, displayed when FAT32
is supported. Be sure to change the "N" to "Y" before proceeding!

Considering how important it is, FDISK is a rather primitive program. It works, but it's cryptic and hard to use. Anything you can do in FDISK you can do more flexibly and easily using a third-party program like Partition Magic. FDISK will not allow you to select or change cluster sizes, resize partitions, move partitions, etc. FDISK's primary advantage is, of course, that it is free (well, built-in anyway).

There is one other option for FDISK, which is undocumented--Microsoft doesn't tell you about it, and it doesn't even show up if you type "FDISK /?". This is the "/MBR" option. If you run "FDISK /MBR", FDISK will rewrite the code in the master boot record (MBR), while leaving the partitions intact. This can be useful for eliminating some types of viruses that infect the master boot record. However (and there's always a however, isn't there?) it can also cause problems in some situations. For example, some viruses encrypt certain disk structures, and if you run FDISK /MBR you may have a more difficult time recovering from the infection. As always, backups are prudent (but don't overwrite ones created prior to the virus in such an instance!)

Warning: Be careful before using the FDISK /MBR command. It is a good idea to do this only if it is specifically recommended for fixing a particular virus or other problem.

Finally, note that Windows NT and Windows 2000 don't use FDISK. They make use of a program called Disk Administrator to handle disk setup tasks. In essence, this is an enhanced version of FDISK that allows you not only to manipulate partitions, but also access some of NT's unique disk management features. For example, you can set up software RAID using the Disk Administrator. See the section on NTFS for more details.


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