MAXIMUM USABLE NUMBER
OF MBR PARTITIONS

(USING MS / IBM MBR PARTITION FORMAT) 


The Maximum Usable Number Of MBR Partitions Is . . .

One of the things that I wondered about when I was exploring multibooting Windows was exactly how many partitions it's possible to have using the MBR partitioning scheme, that is, the total number of usable MBR partitions, both primary and logical.

I started to consult Dr. Web and the thing that stood out the most was that no-one seemed to know what the total number of MBR partitions is.

There are all kinds of very authoritative statements in various forums, the three most common being:
  • "A maximum of 4 partitions", or
  • "3 primary partitions plus some logical partitions/drives", or
  • "Unlimited", "100", "34", pick a number . . .
I was a bit surprised at two things - the fact that no-one seemed to know the exact number, and the fact that a lot of people who were saying "A maximum of 4 partitions" were on forums frequented by techo nerds and/or Linux fanboys (loudly confirming the Dunning-Kruger effect) who were not backward in declaring their answer was the right one and if you disagreed you were certainly a bumpkin who deserved to be insulted or ridiculed.

So I started doing some research and finally found the answer from Microsoft - and verified this by doing a bit of playing around in VirtualBox.

Caveats:

  • There are a number of variations to the MBR partitioning scheme around, but here I'm only referring to the MBR structure as originally intended for the IBM PC (and any later updates).
  • The number here is only relevant for the Microsoft usage of MBR for DOS and then Windows.
  • The number here excludes specialist MBR partitioning schemes such as the Extended MBR partitioning method used by BootitNG / Bootit Bare Metal or other specialist partitioning software.
  • The number here is the total number of partitions that are accessible and usable on a single hard disk, as while it's indeed possible to create more logical partitions than the number stated, all such partitions are unusable.
  • The number here is as verified using Windows XP Disk Management.
  • The pedantic techo nerd / Linux fanboy argument that a logical drive is not a partition is irrelevant and therefore ignored (if you want to have that argument, have it with the person in your bathroom mirror).

And the maximum number of usable MBR partitions is:  26  =  3 Primary + 23 Logical  (on a single hard disk).

(The pedantic
techo nerd / Linux fanboy argument that the true number is actually 27 (3 primary + 1 extended containing 23 logical) is irrelevant and therefore ignored - go argue with the person in your bathroom mirror).


As Microsoft states in numerous sources:

    "An MBR drive can have up to four standard partitions. Typically these are designated as primary partitions."
     and
    "You can designate one of the four standard partitions as an extended partition. An extended partition is a        special partition that can be divided into additional partitions that are called logical partitions (or logical            drives)."


And then further elaborates on the number of extended partitions in Microsoft Q41189 of December 2000:

    "The maximum number of logical drives that can be contained in an extended partition is 23."
      and
    "The maximum number of total drives that MS-DOS can use is 26:  Drive A through Drive Z."

An archive of the original Microsoft KB article Q41189 is here:
     https://github.com/jeffpar/kbarchive/blob/master/txt/041/Q41189.TXT

A later revision to this article (May 2003 Revision 2.0) is here:
     http://web.archive.org/web/20110123010334/http://support.microsoft.com:80/kb/41189

You can also download a .pdf of the December 2000 KB article by clicking here.


So there you go . . . well, kind of . . . see the "Some Quirks" comments below.


And a small aside on the risks of allowing IT graduates to edit your technical articles.

The Microsoft Windows Hardware Developer article dated 30 November 2021 ( link: https://learn.microsoft.
com/en-us/windows-hardware/manufacture/desktop/configure-more-than-four-partitions-on-a-biosmbr-
based-hard-disk?view=windows-11
) now states:

    "You can include as many logical partitions as your disk can hold."

This is obviously not correct - refer to Microsoft Q41189 of December 2000 or KB41189 of May 2003 Revision 2.0 (if in doubt, try it yourself in a VM).

It's unfortunate that the author / editor of this technical article had not verified the correctness of what was being published under the name of Microsoft.

Perhaps it's time to stop having IT graduates edit your articles - or perhaps IT now = I'm on Tiktok ?



Some Quirks Related To 26 MBR Partitions

Well, really one quirk relating to 26 partitions and a mis-understanding relating to the maximum of 26 drives.


The quirk relating to a maximum of 26 MBR partitions:

As I discovered in testing, it is possible in Windows (XP and I assume earlier / later versions) to create more than 23 logical MBR partitions (and therefore more than 26 total MBR partitions) using Windows Disk Manager.

Refer to image below with 24 logical partitions.

More than 23 logical partitions can be created, but trying to access logical partitions above 23 will return a "Disk in drive x is not formatted" error when you try to access that drive, and it also can't be formatted.

What the hey . . .  I've found a bug in Windows - get Bill on the phone.

It looks as though the check to prevent > 23 logical partitions being created has gone walkabout.

So that's the quirk.


The mis-understanding relating to a maximum of 26 drives:

The other annoyance during researching the maximum number of partitions was the constant declaration by experts that Windows can / will / only ever mount a maximum of 26 drives.

I suspect this is caused by the mis-understanding that as only 26 drives can be assigned a driver letter of A through Z, it therefore follows that the maximum number of drives that can be mounted is 26.

This view is probably also compounded by conflating MS-DOS with MS Windows when they're two rather different things.

As stated in KB41189 "The 
maximum number of total drives that MS-DOS can use is 26" - and this restriction does not apply to later versions of Windows that are using the NTFS file system.

When Windows uses the NTFS file system you can choose to map and mount a virtually unlimited number of drives in Windows - the restriction being that only 26 of the drives (i.e., partitions or CD ROMs or floppy drives) can be assigned a drive letter of A through Z.

Mounting drives when you've run out of drive letters is simple - map the drive (partition) to an NTFS folder and Windows will mount
it as a folder and put a nice little hard drive icon against it to indicate it's a drive mounted as a folder.

Refer to the image below where there are 32 drives mounted - with 6 mounted as folders in C:\.

So while it's correct to say:
  • The maximum number of partitions under the MS / IBM partition format is 26 (well . . . the maximum useable number that is, as qualified by the explanation above and the example shown below)
          and
  • The maximum number of total drives that MS-DOS can use is 26
it isn't correct to say that Windows can only mount 26 drives, as the reality is that Windows - providing it's using the NTFS file system - can map and mount a virtually unlimited number of drives.

Try it yourself in VirtualBox or VMware . . . it isn't rocket science (for all you techno nerds and Linux fanboys having this article read to you, get your carer to google "rocket science").


So now you know how many partitions you can have using MBR and how to mount virtually unlimited drives in Windows when using the NTFS file system.

Who'd a thought Windows could be so much fun.




27 partitions + 1 (2nd) HD + 2 FD + 1 CD + 1 LAN for 32 mounted drives - open in a new tab for a larger view

Note the Windows bug which will let you create > 23 logical partitions but then throw an error when you try to access them and if you select Yes to format, then you'll get a doink and a Windows can't format this drive msg

MBR1




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Updated  26 August 2023      
Shotter_Nail