Does it happen with different USB drives?

Do you have the latest BIOS for your motherboard?

Is something sharing the IRQ with the onboard USB controller?

I vaguely remember a similar issue, suspect this is due to how Dos/Win9x assigns drive letters during boot.

HDD 1 Primary partition will default to C:\
HDD2 Primary partition will default to D:\
HDD1 Extended partition logical drives will get next available letters, eg E:, F:
HDD2 Extended partition logical drives will follow on from above, eg G:, H:
Optical drive will gets assigned a letter last.

Problem is dos (and 9x) sees a USB as a removable hard drive and therefore gets priority over the extended partitions.
However you can assign drive letters in Device Manager from memory? although it'll happen each time you use a different stick.

The PCI card is probably the neatest option, been an external card "breaks" windows logic thinking its a hard drive.

Yes has to do with primary versus extended partitions, and the USB is seen as a Primary when you reboot. Yes you can assign drive letters in Win9x similar to XP.

cyclone3d wrote on 2025-05-14, 20:02:

Does it happen with different USB drives?

Do you have the latest BIOS for your motherboard?

Is something sharing the IRQ with the onboard USB controller?

I have two USB key, the first I have the behavior I reported, the second USB drive the PC stop after the POST, nothing append.

On the POST screen I can see that I have V3.4 of BIOS but on MSI website, last bios update for my system is V3.2. Same information on the web site "The Retro Web". So I think I have the last version.

How can I check if the IRQ with the onboard USB controller is shared somewhere?

chinny22 wrote on 2025-05-15, 02:01:

I vaguely remember a similar issue, suspect this is due to how Dos/Win9x assigns drive letters during boot. […]
Show full quote

I vaguely remember a similar issue, suspect this is due to how Dos/Win9x assigns drive letters during boot.

HDD 1 Primary partition will default to C:\
HDD2 Primary partition will default to D:\
HDD1 Extended partition logical drives will get next available letters, eg E:, F:
HDD2 Extended partition logical drives will follow on from above, eg G:, H:
Optical drive will gets assigned a letter last.

Problem is dos (and 9x) sees a USB as a removable hard drive and therefore gets priority over the extended partitions.
However you can assign drive letters in Device Manager from memory? although it'll happen each time you use a different stick.

The PCI card is probably the neatest option, been an external card "breaks" windows logic thinking its a hard drive.

It's not possible in boot process to disable USB hard disk? I will try to fix letter and see what happen

I saw on my USB extension Card that there is an emplacement for solders pins in order to connect the front panel USB port, Maybe it's the better option here.

Ok i have tried to fix letter drive but it s impossible for hard drive, only for optical drive and removal mass storage but my usb key is detected has a hard drive and a mass storage after reboot. I think to solder a USB mâle 9 pin connector on my PCI extension USB 2.0 card will be the only solution.

Francois5942 wrote on 2025-05-15, 15:53:

I have two USB key, the first I have the behavior I reported, the second USB drive the PC stop after the POST, nothing append. […]
Show full quote

cyclone3d wrote on 2025-05-14, 20:02:

Does it happen with different USB drives?

Do you have the latest BIOS for your motherboard?

Is something sharing the IRQ with the onboard USB controller?

I have two USB key, the first I have the behavior I reported, the second USB drive the PC stop after the POST, nothing append.

On the POST screen I can see that I have V3.4 of BIOS but on MSI website, last bios update for my system is V3.2. Same information on the web site "The Retro Web". So I think I have the last version.

How can I check if the IRQ with the onboard USB controller is shared somewhere?

The second boot screen should show what each IRQ is assigned to and should show some other info... I think.

You will want to pres the pause button on the keyboard when it pops up so you can take note of what it says before it goes away.

May need to hit the esc key to show boot info if a full screen logo is showing up on boot.

There should also be some IRQ settings in the BIOS as well.

I can plug in the same name as the v3.2 BIOS on the MSI site but change the last number to 4 and it lets me download the 3.4 BIOS.

I tried higher numbers and there is a v3.5 and a v3.6 - They are also attached to this post.
https://download.msi.com/archive/mb/bos_exe/6734v35.zip
https://download.msi.com/archive/mb/bos_exe/6734v36.zip

KM4M (MS-6734) V3.4 BIOS Release
----------------------------------
1. This is Award BIOS release

2. This BIOS fixes the following problem of the previous version:
- Set Frame Buffer Size to 64M
- Set Shutdown Temperature to 85 degrees
- System can't resume from s3 after disable SATA function

3. 2004/06/18

KM4M (MS-6734) V3.5 BIOS Release
----------------------------------
1. This is Award BIOS release

2. This BIOS fixes the following problem of the previous version:
- Support K7 Sempron CPU
- Support low speed CPU FAN

3. 2004/08/09

KM4M (MS-6734) V3.6 BIOS Release
----------------------------------
1. This is Award BIOS release

2. This BIOS fixes the following problem of the previous version:
- Fix system will auto power on after power lost.

3. 2004/11/01

Francois5942 wrote on 2025-05-15, 17:04:

Ok i have tried to fix letter drive but it s impossible for hard drive, only for optical drive and removal mass storage but my usb key is detected has a hard drive and a mass storage after reboot. I think to solder a USB mâle 9 pin connector on my PCI extension USB 2.0 card will be the only solution.

Your right, I must of been thinking of mass storge device properties.

This is the "official" way but don't think it'll help as the USB has a primary partition
https://www.betaarchive.com/wiki/index.php?ti … %20the%20device.

Tools exist, but I've never used them myself
Re: How to change partition letter under windows 95? And under windows 98?

chinny22 wrote on 2025-05-15, 02:01:

I vaguely remember a similar issue, suspect this is due to how Dos/Win9x assigns drive letters during boot. […]
Show full quote

I vaguely remember a similar issue, suspect this is due to how Dos/Win9x assigns drive letters during boot.

HDD 1 Primary partition will default to C:\
HDD2 Primary partition will default to D:\
HDD1 Extended partition logical drives will get next available letters, eg E:, F:
HDD2 Extended partition logical drives will follow on from above, eg G:, H:
Optical drive will gets assigned a letter last.

Problem is dos (and 9x) sees a USB as a removable hard drive and therefore gets priority over the extended partitions.
However you can assign drive letters in Device Manager from memory? although it'll happen each time you use a different stick.

The PCI card is probably the neatest option, been an external card "breaks" windows logic thinking its a hard drive.

This sorting is right if you don't use a patched version of IO.SYS. It sorts partitions like:
HDD1 Primary partition
HDD1 Logical drive1
HDD1 Logical drive2
...
HDD2 Primary partition
HDD2 Logical drive1
HDD2 Logical drive2
...
so on

as for me this way looks more convenient.

Optical drives assigned letters by MSCDEX right next to last used letter before loading MSCDEX (if not forced by /L: paremeter)
USB devices in DOS are not accessible without a special driver (I would line to know how if it is possible) so at least in WinME they can not affect loading windows shell at all.

I made pictures of boot screen with and without USB stick plugged, here is the result :

Without USB stick:

With USB stick

No difference on IRQs, but we can see that in the step before launching Windows ME, Bios detect the USB drive.

I've tried to boot computer with the Windows ME rescue floppy and try to see if fdisk detect my USB stick as a hard drive but no :

We can see only C: is detected as a the main DOS partition. When we check the details of logical partitions, the letters are wrong because DATA should be D: and RECOVERY E:

So... we confirm that somethings is going wrong in the boot process of Windows. Btw Windows ME don't use ms dos file system for boot process no? it use registry? Maybe a registry hack is possible?
What is weird is that with my old MSI MS-6340M ver 1.0, I didn't have this issue. The chipset was a VIA also. But it didn't have the ability to boot on USB drive.

I searched in BIOS but it seems there is no option for disabling USB drive in boot process.

Do you think updating BIOS can fix the issue? Usually I flash BIOS only if I can find in release notes a fix for a specific problem I have