Everything I've found so far claims /mbr is undocumented. Issuing the command fdisk /?on MS-DOS 6.22 gives no help so I assume there are no command line switches for fdisk. However this raises another question: why? I have had several instances of not being able to boot from a hard drive or PATA SSD, even after erasing all partitions then recreating, formatting, setting active, and using sys c: or performing a full MS-DOS install. It magically begins working after issuing fdisk /mbr. I just had this occur yesterday on a new 1GB IDE SSD and initially believed there to be a hardware problem. If this command did not exist, how would one get a stubborn device to be DOS bootable?

  • The MBR contains some boot/loader code and a partition table. This means any partitioned disk already has a MBR with code that DOS will leave alone when it updates the partition table.
    – Brian
    Commented Jun 25 at 15:19
  • 1
    This flag comes up a lot on this site. :) It seems to be the magic answer to a lot of completely unhelpful error non-messages. retrocomputing.stackexchange.com/questions/5825/… retrocomputing.stackexchange.com/questions/21586/…
    – knol
    Commented Jun 25 at 17:15
  • 1
    It might be helpful to add what DOS version and environment is used.
    – Raffzahn
    Commented Jun 25 at 18:30

2 Answers 2


Yes, FDISK /MBR is undocumented — it isn’t mentioned in the online help, or in the printed manuals. It wasn’t much of a secret though; it was mentioned in magazines, and in books such as DOS 6.0 Power Tools. Microsoft documented it later on (1999) in nominally Windows-related material.

Not knowing about it wouldn’t cause problems for most users: FDISK on its own also writes the MBR when the partition table is created. So using FDISK as documented to set up a new hard drive would write the MBR properly. But there wasn’t any great way of fixing a corrupted MBR (SYS wouldn’t do it).

As to why it wasn’t documented, I can only speculate. The flag was added in MS-DOS 5.0, which was supposed to be a “user-friendly” release (at least, it was the first release actively marketed to end users). FDISK /MBR can fix things, but it can also break working setups, and perhaps the risk of the latter was considered worse than the benefit of the former. (Of course, MS-DOS already had many other ways of breaking working setups, such as RECOVER.)

  • 8
    Programmers forever add things and forget to tell the tech writer.
    – dave
    Commented Jun 25 at 22:52

fdisk /? should give information about the /STATUS and /X switches unless it's a really old version. The /MBR switch was not listed as part of that but was documented by Microsoft in the knowledgebase.

The MS-DOS Fdisk utility usually updates the master boot record (MBR) only if no master boot record exists. Repartitioning with Fdisk does not rewrite this information.

Fdisk 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.

  • 4
    MS-DOS 6.22 simply returns "Configures a hard disk for use with MS-DOS." when I type fdisk /?.
    – eesz34
    Commented Jun 25 at 15:18

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