"People often recommend the undocumented DOS command FDISK /MBR to solve problems with the MBR.
This command however does not rewrite the entire MBR - it just rewrites the boot code,
the first 446 bytes of the MBR, but leaves the 64-byte partition information alone.
Thus, it won't help when the partition table has problems."
Source: https://www.win.tue.nl/~aeb/partitions/partit … es-2.html#ss2.7
"FDISK /MBR (for all of you who wanted to know!)
No matter what version of FDISK you have (from MS-DOS 5.0 or higher that is), you can always enter the command:
FDISK /? at a DOS prompt, and your Windows machine should respond with:
Configures a hard disk for use with MS-DOS. [ Ah! And Windows too! ]
FDISK [/STATUS] /X
/STATUS Displays partition information. [ A nice safe switch. ]
/X Ignores extended disk-access support. Use this switch if you
receive disk access or stack overflow messages.
So, where's the /MBR switch? Well, that's the reason it's often called an undocumented switch! It's never displayed here; neither are any of the many others...
But these various switches are documented in a number of Microsoft books and web pages... Especially in their Knowledge Base articles
see for example: FDISK /MBR Rewrites the Master Boot Record (Q69013) (which has a nice summary of what this switch does).
Basically, it will overwrite everything that's not part of the 64-byte Partition Table near the end of Absolute Sector 0 (the very first physical sector on a hard disk). But read on...
Over and over again you'll read or hear that FDISK /MBR destroys the Partition Table too! For example, here's a quote from another KB article:
" WARNING: This process will repair the bootstrap code and the 55AA signature by rewriting sector zero, but it will also overwrite the partition table entries with all zeros,
rendering your logical drives useless (unless, that is, the 55AA signature is manually entered using a disk editor prior to performing an FDISK /MBR). "
First, reading the quote in context, shows that this applies only if the Signature ID Word AA55h (or 55 AA as seen in a disk editor) has been corrupted!
But beyond that, I can state that after having made several tests using the newer FDISK programs from Windows 95B, 98, 98SE and ME,
that none of the FAT32 capable FDISK programs will ever zero-out Partition Table data; even if the Signature Word AA55h has been erased!
{You should, of course, test that out yourself and always make a backup copy of your Partition Table.}
However, it's also a proven fact that previous versions of FDISK (such as MS-DOS 6.22), will wipe out the Partition Table data with zero bytes under the same circumstances!
So, it appears that Microsoft and others would rather err on the side of caution instead of telling customers it's OK to use the FDISK from,
say a Win 98 boot disk, without worrying about losing their Partition Table data (if it's used correctly).
CAUTION: After stating all of the above, you still need to understand that FDISK /MBR may seldom be a cure for your own MBR problems! Why?
Because it's almost always the Partition Table data itself that has been changed (such as the indicator showing which partition is Active; i.e., bootable).
And if some type of multi-boot manager (which very often resides on more than just the first sector) should fail, FDISK /MBR might not help at all;
UNLESS you kept a record of all the Partition Table data, and are capable of entering that data back into a Table of ALL zeros.
Fortunately, there are some nice tools available (see below) to do that."
Source: https://thestarman.pcministry.com/asm/mbr/FDISK.htm
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