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I was using the latest version of Rufus (more precisely, Rufus 4.1.2045 Portable) to createa a bootable MS-DOS USB drive. This works well, after booting, the ver command shows the following.

Windows Millennium [Version 4.90.3000]

As I have an old backup of an MS-DOS 6.22 system lying around, I tried to copy the system files (i.e. msdos.sys, io.sys and command.com) to the USB drive. I was hoping to be able to boot into MS-DOS 6.22.

However, this was not the case - instead I got the following error message.

Invalid system disk
Replace the disk, and then press any key

Why did this not work as I expected? I believed that the MBR would just load io.sys and msdos.sys which then proceed to boot from the file system. Apparently things are more involved; what is going on? Is the MBR specific to the version of MS-DOS it belongs to? Do the low-level system files need to be at a specific physical location in the file system?

Any help appreciated.

Edit: Note that the file system is FAT32; could this be part of the issue?

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Generally, the DOS-style MBR only searches for an active (bootable) partition; it's the job of that partition's VBR to load IO.SYS. Such a MBR is somewhat generic, while the VBR is specific to the OS as well as to the filesystem that it needs to understand. If Rufus created your USB stick as unpartitioned FAT32 then the MBR and VBR are the same thing, but I believe usually they're separate (you have one large partition).

As far as I know, the standard DOS or Windows VBR is extremely basic and requires IO.SYS to be the first entry within the FAT root directory; likeewise MSDOS.SYS must be the second entry. In addition, at least according to Wikipedia (and the MS KB it references), the files must be physically located as close to the beginning of the filesystem as possible. If you copied those files by hand, they're most likely in the wrong spot within the filesystem and/or the FAT.

In the past, the sys command would be used to insert these files in the correct location.

(Many other boot sectors have similar requirements; sometimes the bootloader installation involves a program that embeds the physical position of the "next stage" file into the VBR.)

On top of that – yes, MS-DOS 6.22 does not understand FAT32 as far as I know, so even if the Windows ME VBR that's still on your disk were able to boot IO.SYS, the DOS kernel wouldn't be able to find the shell. (There's probably a reason why Rufus uses the Windows ME files, in the end – among other things, they have a newer version of MS-DOS with FAT32 support.)

Until recently, Microsoft used to publish the installation floppy disk images of MS-DOS 6.22 as a free download from their own website (search the web for en_msdos622.exe).

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  • Yes; You very much have to install MS-DOS 6.22 and then install Windows ME, as Windows ME, is not based on the Windows NT kernel.
    – Ramhound
    Commented Jul 25, 2023 at 15:53
  • @Ramhound: NT kernel or not really doesn't matter – the WinME 'Setup' installs its own version of DOS anyway, regardless of what you had previously. From what I remember, starting with Windows 98 (SE?) the install media was already bootable directly, and would automatically partition the disk and install its internal copy of DOS if necessary. (...I believe Win95 floppy disks were bootable as well, it's only Win3.x that required the user to have their own DOS pre-installed.)
    – grawity_u1686
    Commented Jul 25, 2023 at 16:10
  • Didn't you have to boot into DOS then start Windows with Windows ME. That was at least the case with Windows 95 and Windows 98 SE although I must admit I might be thinking of Windows 3.1.
    – Ramhound
    Commented Jul 25, 2023 at 16:23
  • @Ramhound: No, it definitely was not the case even with Windows 95. Maybe in the early Chicago builds, but not in the final release – the system would boot straight to Windows after autoexec.bat (whereas in Windows 3.x you had to run win manually). The same with installation – the Windows 3.x install floppy had to be run from an existing MS-DOS system, while at least Windows 98 already had a directly-bootable install floppy & CD that'd install DOS if needed (...much like MS-DOS's own installer).
    – grawity_u1686
    Commented Jul 25, 2023 at 16:32
  • Thanks for the clarification.
    – Ramhound
    Commented Jul 25, 2023 at 16:34

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