No longer have access to Ubuntu in dual-boot

Support and Help
1

Ubuntu Version:
22.04 LTS

Context and Problem Description:
Tl;dr: BIOS no longer boots into Ubuntu, even though it lists the UEFI OS which should correspond to Ubuntu.
For context, after the end of Win10 support, I decided it was time to switch to Ubuntu. To make the transition easier for me, I decided on a dual-boot system, Win10 remaining on a tiny 128 GB drive, Ubuntu on a more reasonably sized one. Following this video: https://www.youtube.com/watch?v=KWVte9WGxGE, I decided on Dual Drive Method A, which means that I completely disconnected the Windows SSD when installing Ubuntu, since (as I understood it) keeping it connected would force install Grub on the Windows drive, and I seriously didn’t have any room for it. Weeks went by, I had no issues switching between the OS-s, except every time I booted into Windows, it wanted to do a disk check. So far I’ve always pressed a key to skip it but yesterday I stepped away from my computer for too long and the disk check went through, which seemed to have seriously broken something. Selecting the Ubuntu boot SSD (now named simply UEFI OS), the computer restarts. Even more bizarrely, if I try to boot from the same SSD, just not the boot partition, it boots into Windows. I’ve seen folks have similar issues, so following some advice I’d seen, I created a Boot-Repair report via the same live-USB that I used for installation: Ubuntu Pastebin

Relevant System Information:
Using a custom made PC with a Gigabyte motherboard

What I’ve Tried:
I’ve gone through my boot settings, testing all boot options, and nothing so far. Here’s a picture of the boot settings:

20251122_125545
20251122_1255452268Ă—1701 161 KB

Boot option 1 was Ubuntu, but now restarts the boot process; option 2 somehow boots into Windows along with option 3; option 4 doesn’t have an OS on it.

I am very much new to this community and wanted to get some advice before I accept the recommended repairs and risk losing all data on my Ubuntu drive, as I can’t access it through Windows. Any help or advice is greatly appreciated!

2

This was an excellent decision on your part.

However, you have Windows 10 in old fashioned Legacy mode and Ubuntu in modern UEFI mode (indicated by the following lines)

Line 5 - Windows is installed in the MBR of /dev/nvme0n1
Line 285 - Grub-efi would not be selected by default because legacy Windows detected.

Also, in your screenshot, you have enabled CSM Support - this is not required for Ubuntu installed in UEFI mode.

Compatibility Support Module (CSM) is a component in your motherboard’s UEFI firmware that allows for compatibility with older legacy BIOS systems.

In order to check that Ubuntu boots OK, try this:-

Disable CSM support
Disconnect and/or remove your Windows disk
Hopefully, Ubuntu 24.04.3 should boot

First of all, let’s see if Ubuntu then boots successfully?

3

Hi, thanks for the quick reply!
This sadly changed nothing, other than only listing and letting me boot into the P3 boot device, with the same reboot happening every time I try. I know that some people have gotten things running again by disabling secure boot, but I can only do that if CSM is disabled. I’m very hesitant when messing with BIOS and Windows boot stuff since I can’t make a proper backup of my Windows drive (I have 2 GB of free memory on that drive, which seems to be too little to create a full system image onto an external hard drive). Is disabling secure boot worth a try?

4

Windows cannot access ext4 filesystems

Only have your Ubuntu disk attached
Boot into a “Try Ubuntu” live session (in UEFI mode)
Back up your Ubuntu personal data

I’ve just noticed that the boot files for the Ubuntu ESP (sdb1) are also missing

You should have these:-

/efi/BOOT/fbx64.efi /efi/BOOT/mmx64.efi 
/efi/ubuntu/grubx64.efi /efi/ubuntu/mmx64.efi 
/efi/ubuntu/shimx64.efi /efi/ubuntu/grub.cfg

While in the live session, install boot-repair again
Run the recommended repair on your Ubuntu disk only
Your back up is now your safety belt

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5

Awesome, this seemed to have worked! Now to make sure Windows isn’t doing an automatic disk check any time I decide to boot into that instead of Ubuntu

6

The main complication in your set-up:-

  • Windows in Legacy mode with msdos partition table
  • Ubuntu 24.04 in UEFI mode with gpt

Separate disks for each OS is allowing a bit of latitude
You can continue like this as long as you are very, very careful

Having been tripped up once by Windows (chkdsk), you may wish to consider:-

  • Back up your Windows personal data
  • Install Windows 11 in UEFI mode with GPT (Windows 10 is now EOL)
  • Still preferable to have separate disks for each OS
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7

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8

One more suggestion.

Your sda & sdb seem to be HDDs which are fine, but slower that NVMe drive you also have. The NVMe drive shows NTFS, so is it just Windows data?

If converting to Ubuntu best to convert all data to ext4 or other Linux formats. While Linuc can read & write to NTFS, it cannot fully repair ntFS. So as long as you have NTFS you need Windows.

And since NTFS is so much faster, better to have install on the NVMe drive and data which is used less on the slower HDD. Many also consider HDD better for backup.

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