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Hardware Emergency Room:   FDISK and FORMAT
Contents: Using the FDISK Utility

Using the FDISK Utility

Warning:   FDISK is an extremely powerful program.  If you delete a disk partition by using FDISK, all the data in that partition will be permanently destroyed.

Basic Steps Required to Configure a Hard Disk

The following steps are required if you want to upgrade your existing hard disk, add a new hard disk, or reconfigure your current hard disk because it is corrupt or unreadable.

1.  Create a start-up (or emergency boot) disk by following our simple directions for making a boot disk, using the Add/Remove Programs option in Control Panel, or use a start-up disk such as that supplied with various commercially available utility programs

2.  Back up important files on the hard disk.

(Note Step 2 is only necessary if there are files on the disk that you need or want to save.)

3.  Partition the hard disk using FDISK.

4.  Format the hard disk using the Format command.

5.  Restore any backed-up files.

FDISK is an MS-DOS-based application that can be run from an MS-DOS command line.   The partitions that FDISK creates are called MS-DOS partitions.  Your start-up disk contains a copy of FDISK.

To Start FDISK

1.  To start FDISK from a start-up disk, put the disk in drive a: and press CTRL+ALT+DEL to restart the computer.  At the command prompt on the a: drive, type FDISK.

2.  If you have a hard disk smaller than 512 MB, the FDISK Options screen appears.   You can choose to do the following:

  • Create a partition or logical drive.
  • Set the active partition.
  • Delete a partition or logical drive.
  • Display partition information.
  • If the computer has two or more hard disks, FDISK displays a fifth option named Change Current Fixed Disk Drive, with which you can switch to another disk drive.

– Or –

If you have a hard disk larger than 512 MB, the following screen appears:

fdisk.gif (4688 bytes)

Important If you enable large disk support, all drives created will be FAT32.  As such, you will not be able to access the newly created drive if booting from a boot disk created by a previous version of Windows 95, Windows 95A, or MS-DOS. Windows 95B (OSR2) does support FAT32, however.  In addition, legacy third-party disk utilities may not function on FAT32, so it is highly recommended you contact your software vendor to ensure FAT32 compatibility.  Many third-party manufacturers already have FAT32 versions of disk utilities available.  Utilities included with Windows 98 are compatible with FAT32.

If FDISK identifies that you have a drive larger than 512 MB it asks if you wish to enable large disk support, so that your system can use FAT32 as your file system.  You can choose one of the following options:

  • Answering “Y” means you will be using a 32-bit FAT. FDISK can make the entire disk available as one partition (up to 2,047 GB).
  • Answering “N” means you will be using a 16-bit FAT. FDISK will only allow up to 2 GB for a partition even if the disk is larger.

To Configure a Hard Disk Using FDISK

1.  Delete all MS-DOS partitions in the following order:

  • the logical drives
  • the extended MS-DOS partition
  • the primary MS-DOS partition

2.  Create a new primary MS-DOS partition.

3.  (Optional) Create an extended partition and logical drives.

Caution If you use FDISK to repartition a hard disk, all the files on the original partitions will be deleted. Be sure to back up all data files on the hard drive before using FDISK.

Deleting Partitions and Logical Drives

You can use FDISK to delete partitions before creating a new primary partition.   You must delete partitions in the following order:

1.  Any non-MS-DOS partitions.

2.  Any logical drives in the extended MS-DOS partition.

3.  Any extended MS-DOS partition.

4.  The existing primary MS-DOS partition.

To Delete a Partition or Logical Drive

1.  In the FDISK Options screen, press 3, and then press ENTER. The Delete DOS Partition Or Logical DOS Drive screen appears.

2.  Press the number for the kind of partition you want to delete, and then press ENTER.

3.  Follow the directions on the screen, and repeat the steps for deleting any additional logical drives or partitions.

Creating a Primary MS-DOS Partition

On a new disk, or after you have deleted a primary MS-DOS partition, you can create a new primary MS-DOS partition.

To create a primary MS-DOS partition:

1.  In the FDISK Options screen, press 1, and then press ENTER. The Create DOS Partition Or Logical DOS Drive screen appears.

2.  Press 1, and then press ENTER. The Create Primary DOS Partition screen appears.

3.  If you want the partition to be the maximum size, press ENTER. Then insert a start-up disk in drive a:, and press any key.

– Or –

If you do not want the partition to be the maximum size, press n, and then press ENTER. Another Create Primary DOS Partition screen appears.

4.  To specify the partition size you want, follow the instructions on-screen, and then press ENTER.

You can specify the partition size as a percentage of disk space or in megabytes of disk space. If you specify a percentage of disk space, include a percent sign (%) after the number.

5.  If you create the Primary partition to use the entire hard drive, press ESC twice to exit FDISK, then reboot the computer to the floppy disk.

If you chose not to use the entire drive for the Primary partition, you need to create the Extended DOS partition (unless you plan to use the remaining disk space for other reasons, such as a different operating system).  To do this, select Option 1 from the main FDISK options screen., then choose to create an Extended DOS partition.  This will typically be set up for the remaining disk space on the drive. You will be prompted to create logical drives upon establishing the Extended DOS partition.  Specify the partition size you want as a percentage or number of megabytes of disk space.  Select logical drive sizes the same way.

Finally, if you created a Primary and an Extended partition, you will need to set an active partition.  From the main FDISK options menu, select option 2, and then select the Primary partition to set active.

Verifying Drive Integrity

While using FDISK, each time you create a partition or logical drive, FDISK displays the following message on the bottom of the screen:

Verifying drive integrity, --% complete.

The percentage counts 0 to 100. When verifying the drive’s integrity, FDISK is checking the tracks where the system files and FAT will be stored.  FDISK displays this message since it has capabilities for setting up very large drives and the process may take some time.

The check initially verifies the number of tracks necessary to hold the system files and FAT as if the entire disk were partitioned as one primary partition.  If you create an extended partition it will verify the number of tracks necessary to hold the system files and FAT as if the remaining portion were partitioned as one logical drive.  Finally, with each logical drive created, FDISK verifies the number of tracks necessary to hold the system files and FAT as if the remaining portion of the extended partition were defined as a logical drive.  This repeats until you finish creating logical drives.


At the end of the ROM BIOS bootstrap routine, the BIOS reads and executes the first physical sector of the first floppy or hard drive on the system.  This first sector of the hard drive is called the master boot record.

The FDISK command has an undocumented parameter called /MBR that causes it to write the master boot record to the hard disk without altering the partition table information.   Note that the /MBR command works only on the first hard disk on the computer.

Warning: FDISK /MBR should only be used as a last resort.  It may render the files on your drive inaccessible.

Warning: Writing the master boot record to the hard disk in this manner can cause problems for dual-boot programs, systems that use third-party disk partitioning programs, and disks with more than four partitions.  DO NOT use this command if you have a Windows NT system in which the disk contains partitions or logical drives that are members of mirror sets, stripe sets with parity, volume sets or stripe sets.  If you have none of these volumes /MBR can safely overwrite the disk signature.  After overwriting the signature with /MBR you should then use Disk Administrator to write a new signature.

If your computer's master boot record becomes corrupted a new master boot record can be written to the disk using the following command:


This replaces the master boot record without altering the partition table at the end of the sector.  There is no message or response to this command.


Warning:   FORMAT is an extremely powerful program.  When you use FORMAT on your hard disk it destroys all data on the disk.  Make sure you backup any critical data prior to using FORMAT.

The hard disk on your computer must be formatted before you can run Windows Setup.

To Format a Hard Disk drive Using a Windows Start-up Disk

1.  Make sure a start-up disk is in drive a: and press CTRL+ALT+DEL to restart the computer.  Then, at the command prompt, type the following:

format drive:

For drive, type the letter of the drive you want to format.

If you are formatting drive c:, copy system files to the hard disk by typing the following at the command prompt:

format c: /s

When the warning message appears, proceed with formatting by pressing Y.  Then press ENTER.

2.  When formatting is complete, type a volume label (if you want one), and then press ENTER.

3.  If you want to format other drives, repeat steps 1 and 2.

4.  Remove the floppy disks from all floppy disk drives, and restart the computer by pressing CTRL+ALT+DEL.

Tip: If your DOS formatting seems to take too long, check your DOS version to make sure you are running MS-DOS version 6.22 (or MS-DOS 7.0 with Windows 95).   Earlier versions include hard-error retries which significantly add to format time.

Disclaimer:  These pages are provided for information purposes only.  We cannot be held responsible for any damage you might inflict on your system while using the information contained herein.  We recommend you always refer any technical matter that is "over your head" to a qualified computer technician.

© 2011 Double-Hammer Computer Services.  All rights reserved.
Last Updated: September 04, 2011

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