Black screen when loading in Legacy Boot Mode

[Opinin]

Hi,

My PC Specs:
Intel Core i5-10400 (BX8070110400) processor
Gigabyte B460m DS3H motherboard
DDR4 8GB (2x4GB) 2666 MHz HyperX Fury Black Kingston (HX426C16FB3K2/8) memory

I've problem to boot from bootable USB in Legacy Boot Mode.
The "CSM support" option was turn to "Enabled" inside the UEFI setup.
I selected removable device without "UEFI:" prefix in boot menu and then saw black screen with white underscore at the top left corner of the screen.
The same result was with plugging different displays to VGA, DVI and HDMI ports on the motherboard.
Please help me understand, why the integrated GPU in the CPU don't outputs the video to the display?
When I selected removable device with "UEFI:" prefix in boot menu then loading was ok.

[n_scott_pearson]

What kind of SSD are you using? If NVMe, you *must* use UEFI Boot; you *cannot* use Legacy Boot.

...S

[Opinin]

Thanks for answering me.

SSD and HDD are disconnected from the motherboard. The only USB flash drive with installed OS is connected to boot.

[n_scott_pearson]

Ok, nothing obvious coming to mind. You need to contact Gigabyte and discuss their BIOS implementation.

...S