Lubuntu dual boot installation with hybrid EFI motherboard

I am trying to install Lubuntu 26.04 on an old computer, dual boot alongside Windows 10 on the same GPT SSD, however after a successful installation the computer boots into Windows. Grub does not appear, and I have no option to boot into Linux.

The motherboard is a Hybrid EFI Gigabyte 78LMT. Apparently it can make dual booting very tricky, but I lack the technical knowledge to understand details like these. Windows is installed in UEFI mode, but I can only boot from USB in legacy mode; hence, Lubuntu is not installed in EFI mode. The inability to boot USB in EFI mode is a known issue with this motherboard and its "hybrid EFI" technology.

Specs:

• Motherboard: Gigabyte GA-78LMT-USB3 Rev. 5.0

• CPU: AMD FX-8350 (8-core, 125W, AM3+)

• RAM: 8GB DDR3 1600 MHz (2×4 GB)

• GPU: AMD Radeon HD 7750 1GB GDDR5

I have tried this solution but boot-repair gives the following error:"The current session is in Bios-compatibility mode". I cannot setup the BIOS to boot USB in EFI mode, the option exists for CD/DVD only, as explained here:

Although Gigabyte's Hybrid EFI contains a full-fledged EFI implementation from an OS's perspective, the firmware setup utility provides almost no options to help you use this functionality. Specifically, only one firmware option, entitled "EFI CD/DVD Boot Option," on the "Advanced BIOS Features" menu, relates to this feature.

In another question, someone tried to install from CD, apparently without success. I have no CD/DVD drive to test this option.

I cannot find any UEFI settings to select Ubuntu as the default operating system to boot either; thus, it seems this solution will not work: no UEFI settings.

I can choose from which disk to boot; Windows and Ubuntu are on the same SDD. I have also an old HDD I use for backup attached to the motherboard; perhaps installing GRUB on it could be a solution?

This is the workaround I found after spending a lot of time on this issue. It is not a perfect solution, but it does the job of having the two OS on this device. But it seems that these Gigabyte Hybrid EFI motherboards make dual-boot extremely complicated. I was unable to get GRUB to work with both Windows and Lubuntu.

Since I have another HDD I use for backup, I created a partition for GRUB there and installed GRUB in Legacy mode grub-install --target=i386-pc.

I left Windows intact on the main SSD. Configuring GRUB to load Windows does not work: the PC restarts.

If I want to boot into Lubuntu, I choose to boot from the HDD. If I want to boot into Windows, I choose to boot from the SSD. In the Bios settings, I can set from which drive it will boot by default.