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Notes on FDISK's
Undocumented Switches


A Guide to using the command line switches in FDISK

The "FDISK /MBR" command is found on our "Detailed Notes about FDISK" page since it's the one most people want to know about.

Index to this page:


Any data presented here that has not been thoroughly tested in house will be marked as "Untested" by us; however the data as presented here should still be fairly reliable.

CMBR   ( Windows 95B/DOS 7.1 or later )

Entering FDISK /CMBR at a prompt, causes FDISK to display:

   You must specify a drive number with /CMBR.
 The master boot code has NOT been updated.
This switch is used to overwrite the MBR code just like /MBR, but on a second, etc. hard disk rather than the primary one. Example:

writes the MBR code to the first sector (Absolute Sector 0) of the Second physical hard disk on a system. ( Entering: FDISK /CMBR 1 is the same as using an FDISK /MBR )

XMBR   (Windows 98SE or Windows ME) Untested.

?  This switch might be a combination of the "/X" and the "/MBR", but we do not know that for sure (it's only a guess); we've been unable to find anyone else who knows either.
Does ANYONE KNOW FROM EXPERIENCE WHAT THIS ONE DOES?    Please send us a link to any documentation.


Q   (already in DOS 6.2 )

This switch was used in older MS-DOS versions to supress the mandatory ( forced ) reboot after making changes to the Partition Table.

Inclusion of this switch in versions beginning with Windows 95B is simply for backwards compatibility with any possible Batch files, that may still be in use from earlier MS-DOS days. Since the introduction of Win 95B (OSR2 or MS-DOS 7.1), the /Q switch can be thought of as a cosmetic option since FDISK will function exactly the same way with or without it... FDISK no longer forces a reboot even when you're operating at a real DOS prompt prior to booting the Windows GUI.

As an example: After using a disk editor to set the Primary partition of the first hard disk to be "non-bootable" (something you cannot do with FDISK), I entered FDISK at a real (16-bit) DOS prompt and was greeted with:

                             Microsoft Windows 95
                           Fixed Disk Setup Program
                   (C)Copyright Microsoft Corp. 1983 - 1995

                                FDISK Options

   Current fixed disk drive: 1

   Choose one of the following:

   1. Create DOS partition or Logical DOS Drive
   2. Set active partition
   3. Delete partition or Logical DOS Drive
   4. Display partition information
   5. Change current fixed disk drive

   Enter choice: [1]

   WARNING! No partitions are set active - disk 1 is not startable unless
   a partition is set active 

   Press Esc to exit FDISK
After selecting choice [2] in order to navigate to the Set Active Partition display, we see:
                          Set Active Partition

   Current fixed disk drive: 1

   Partition  Status   Type    Volume Label  Mbytes   System   Usage
    C: 1              PRI DOS   WIN95_C_DRV   3016   FAT32      100%

   Total disk space is  3016 Mbytes (1 Mbyte = 1048576 bytes)

 Enter the number of the partition you want to make active...........: [ ]
Entering a 1, gave this display:
                          Set Active Partition

   Current fixed disk drive: 1

   Partition  Status   Type    Volume Label  Mbytes   System   Usage
    C: 1         A    PRI DOS   WIN95_C_DRV   3016   FAT32      100%

   Total disk space is  3016 Mbytes (1 Mbyte = 1048576 bytes)

   Partition 1 made active

   Press Esc to continue
After which, pressing the ESC key produced a beep and the following display:

     You MUST restart your system for your changes to take effect.
     Any drives you have created or changed must be formatted
     AFTER you restart.

     Shut down Windows before restarting.

     Press Esc to exit FDISK

The note seems a bit odd since I never booted into the Windows GUI, but we do get the point. Anyway, the reason for this is that it allows you to carry out some other task(s) before you have to reboot the system; something obviously necessary if you ran FDISK from within the Windows OS.



X   ( Windows 95B or later ) Untested, but "official".
Disable LBA Partition Types switch.

By default, FDISK will use the special “LBA” Partition Types when a partition goes beyond (or includes) the 8.4 GB capacity-point on the drive. Using this switch disables these special types, and may also limit the full use of a drive having a capacity greater than 8.4 GB.



EXT   (already in DOS 6.2 ) Untested.
Create Extended Partition
/EXT: <size in MiB> <HDD#>

Creates an Extended Partition of the specified size and on the specified hard drive. NOTE: An Extended Partition by itself can not be formatted; it's only a “container” for one or more Logical Volumes (see the /LOG and /LOGO switch options below).


PRI   (already in DOS 6.2 ) Untested.

Create Primary Boot Partition (using Primary DOS type) and set “Active”  Creates a Primary Boot partition of the specified size, on the specified hard drive (both values required). The resulting partition will be set as the “Active” (bootable) partition on the specified hard disk (secondary disks are not, of course, bootable).
[ Volumes of 512 MiB or larger will automatically become FAT32 when using the latest FDISK program. ]

/PRI: <size MiB> <HDD#>


PRIO   ( Windows 95B/DOS 7.1 or later ) Untested.

Create Primary Boot Partition as FAT16 only and set “Active” — Creates a Primary Boot partition of the specified size and on the specified hard drive, but only as a FAT16 partition-type. In effect, this option will disable FAT32 functions in newer FDISK utilities when creating the partition.

/PRIO: <size MiB> <HDD#>



LOG   (already in DOS 6.2 ) Untested.
Create Logical Volume when creating Extended Partition.
/EXT: <size MiB> <HDD#> /LOG: <size MiB>

Creates a Logical Volume of the specified size, however, it must be used when creating the Extended Partition. Volumes of 512 MiB or larger will automatically become FAT32 when used with the latest MS-FDISK program. ( See notes for /EXT above. )


LOGO   ( Windows 95B/DOS 7.1 or later ) Untested.
Create Logical Volume as FAT16 only (when creating Extended Partition).
/EXT: <size MiB> <HDD#> /LOGO: <size MiB>

Creates a Logical Volume of the specified size, forcing it to be a FAT16 volume type. However, it must be used when creating the Extended Partition (see notes for /EXT above).



ACTOK   ( Windows 95B/DOS 7.1 or later ) Untested.
Disable Hard-Disk Integrity Check

By default, FDISK will perform a disk-integrity scan before creating a partition or volume (it checks the first track of each cylinder on a drive; which shouldn't take long at all, so do not use this switch if you think it will save a lot of time). Using this option will cause FDISK to skip this scan, but also invites the possibility of using a drive that contains bad sectors in the “system area” (first track) of a partition. Use of this switch definitely affects the way that FDISK normally operates!



FPRMT   ( Windows 95B/DOS 7.1 or later ) Untested.
Prompt for FAT16/32 on each partition

   By default, FDISK prompts for “Large Disk Support” whenever a hard-disk of greater than 512 MiB is detected. But this will only enable FDISK to use FAT32 partition-types when the partition-size itself is also greater than 512 MiB. By using this option, FDISK will prompt you to use FAT16 or FAT32 types for each partition or volume that is created during that session, regardless of size.
[ Because this option will over-ride the ">512MB" convention, using it can give unpredictable results. Use with caution. ]



/PARTN   ( This is a switch for the commands MIRROR and/or UNFORMAT under MS-DOS 5.00 through 6.22 and IBM DOS 5.00 or higher ).
This is NOT a switch for the FDISK command.

So why is /PARTN on this page? Well, I've found a number of sites on the Net that state, contrary to fact, that this is a secret FDISK switch; but it's not !!  When we recently found this in an otherwise excellent book on PCs, we were a bit shocked and determined to do our part in removing this faulty info from the Net. Please direct the webmasters of sites with the erroneous information to our statement here:

Notable fact: the /PARTN switch (and the PARTSAV.FIL file) were created by the authors of PC Tools, Central Point Software, Inc. (not Microsoft®) and were first introduced as a part of DOS under MS-DOS 5.00 with the following copyright notice:
"MIRROR, UNDELETE, and UNFORMAT Copyright (C) 1987-1991 Central Point Software, Inc." So you can be sure that they were never a part of DOS until that time.

And for the record, we also did a search through IBM® versions 1.00, 1.10, 2.00, 3.00, 3.10, 3.30, 4.01, 5.00.1, 5.02, 6.10, 6.30, 7.00 (2000) and Microsoft's® 2.00, 2.05 (yes, for the DEC RAINBOW), 3.10, 3.30, 3.30A, 4.00, 4.01, 5.00, 6.0, 6.20, 6.21, 6.22 plus a few odd versions of DR-DOS, etc. and can say without doubt that the /PARTN switch only appears in the commands MIRROR and UNFORMAT for MS and IBM DOS 5.00 and at least in the UNFORMAT command alone for both Microsoft and IBM versions of DOS through MS-DOS 6.22 and IBM's DOS 6. Microsoft seems to have dropped MIRROR from some of its 6.xx versions, and IBM didn't even include the /PARTN switch with some of its DOS 6.xx versions of UNFORMAT.

Therefore, without any doubt, the /PARTN switch has never been used with the FDISK command.

So, how did this erroneous statement come about? Since the FDISK command has had only one (and then later on two) official switches (/STATUS and /X) for its entire history, we don't think any DOS manuals created this error! We believe this erroneous information can be traced to someone who read one of the many pages on the Net that discuss both FDISK and MIRROR commands in the same section, and then made the mistake of confusing which of those commands used the /PARTN switch when copying the data to one of their own web pages! Most of the sites that are in error today, are probably due to people copying the first erroneous web pages without ever verifying the data. I've also seen a number of sites that have copied the same page (including the same exact spelling and grammar errors!). This whole problem is rather similar to the study of how errors were transmitted by scribes who copied ancient documents; something I've studied on occasion.

In conclusion: To guarantee accurate information, you must verify data with many good and trustworthy sources (at least three as a bare minimum; but you should always use quite a few for numerical data). And make sure none of them simply copied from one of the other sources!

Oh, by the way, there are much better tools today for saving and restoring the Partition Table than trying to use the old MIRROR/UNFORMAT commads in DOS! For example, you can read about the MBRWizard on my Boot Tools page here.


Last Update: 24 May 2004.

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