HP Computer boots to GRUB prompt, but it will boot to GRUB menu after F10, how to fix?

I am stumped by this issue:

I have installed Ubuntu on an UEFI HP machine. On the first reboot after the install all went well and the GRUB bootloader menu dialog appeared as it should, allowing for a dual boot with Windows 10. However, after booting Ubuntu and doing a system update and installing some NVidia software the next reboot I had to go through some MOK hoops (Machine owner Keys) brought up by BIOS, perhaps(?). All seemed well until I had to reboot. Here I was met with a dreary grub prompt on a solid black screen.

However, I reboot and went in to UEFI BIOS and reordered the Boot Order, placing Ubuntu over the Windows Boot Manager.

The system booted uneventfully to the GRUB bootloader menu and Ubuntu was launched. All was fine until the next system boot, where again I was met with the grub prompt.

The physical system is based on two disks: one SSD with Windows 10 pre-installed and one 1TB drive with a 200 BG partition for Windows 10 work, and the rest for Ubuntu. The EFI partition with all the boot files exists on the SSD drive. I tried to remedy the boot problem by running a grub update command within Ubuntu, which reported that all was executed successfully. But the problem with booting straight to a grub prompt persisted.

I should note that I found a second but empty partition on the harddrive at /dev/sda2 - but this is not listed in the BIOS boot order list. Thinking that this was a source of confusion for BIOS, I copied the boot directories and files from the SSD partition to /dev/sda2, but this only caused aggrivation when checkin BIOS Boot Order again and finding them to appear there, doubling up everything - these were later deleted so that EFI partition at /dev/sda2 is empty again.

So why does the computer boot to a grub prompt when powered on, but it will boot to a GRUB Bootloader Menu ONLY after paying a quick visit to BIOS - exiting without changing anything - after hitting F10 while powering up?

(I've scoured through many previous postings, but the great majority deal with MBR and not GPT boot-tables, and I am interested keeping Secure Boot intact.)

Cheers,

Gustav

I have solved the problem I had with the computer going directly to the grub prompt immediately on start-up:

I realised that what was actually happening was that the computer booted directly to the grub prompt within a few seconds after pressing the power-on button. However, if I visited BIOS only for a brief moment, then the computer would go through the whole power-up process, i.e. showing the BIOS HP screen for a few more seconds and then finally I would see the GRUB bootloader menu. The system I created was in fact quite healthy, with the Secure Bootloader files intact and in the proper partitions.

So I had to slow down the start-up process somehow, in order for the start-up process to read the relevant boot files. This machine has no "Fast Boot" to turn off in BIOS, something I had already looked for while trouble-shooting, BUT it does have an Advanced -> Power-On Options section in the BIOS: I found POST Delay (in seconds) and set this to 1o

Then every power-on after this went straight to the Grub Bootloader menu.

I hope this helps anyone else trying to trouble-shoot similar issues.

Cheers! Gustav