I am setting up a new development computer with FreeDOS and can boot off the SATA or USB drive. This is my first new computer with sophisticated UEFI (I have CSM enabled).

If I boot from the USB I will see the USB as C: and the SATA as D:. However if I boot off the SATA, DOS does not see the USB drive.

I suspect the FreeDOS USB driver is not doing what it is suppose to but it seems wrong that DOS sees it if it was used for a boot?

Has anyone experienced this problem and found a solution?

  • What “FreeDOS USB driver” are you using? Commented Jan 21, 2019 at 5:38
  • I'm voting to leave this open because FreeDOS is Retrocomputing in my opinion. Commented Jan 21, 2019 at 10:40

1 Answer 1


The FreeDOS kernel doesn’t support USB drives on its own.

When you boot from a USB drive, the CSM makes it available through the BIOS 13h services, so it appears to DOS as a “standard” drive and everything works fine.

When you boot from your SATA drive, the CSM doesn’t set anything up for your USB drive (although in some cases it can be induced to do so) and DOS doesn’t see it. To access the drive in that case, you need to install and configure a USB driver; FreeDOS provides the usbdos package for this purpose.

  • As an interesting update I have found that if I have a non-system USB stick in a USB port during boot DOS can access all USB ports after that. However it will not detect any other usb stick that was not in at boot. It seems the BIOS will remap the stick (perhaps keeping track of its serial number) as I move it from port to port... weird!
    – jwzumwalt
    Commented Jan 21, 2019 at 8:49
  • 1
    And CSM is?.... Commented Jan 22, 2019 at 22:04
  • @Thorbjørn Compatibility Support Module, an optional module in UEFI which emulates BIOS services. Commented Jan 22, 2019 at 22:17
  • It's not just an issue with UEFI, traditional BIOSes may or may not configure a USB drive as a DOS accessible 13h storage device when you don't boot from it, even though it does when booting from it. Commented Sep 8, 2020 at 3:07

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