I have an HP Elitebook 8460p from around 2012 which had Windows 10 on it. The BIOS is set to legacy, since UEFI is offered only for development purposes, with big warnings not to use it otherwise.
When I used a USB drive to "erase the disk and install" Ubuntu 21.10, everything went fine until the reboot, where I got "no operating system found" (error: 3F0). I checked with gparted and the disk was GPT with an UEFI partition.
This is strange, since Windows was likely in Legacy mode, given the BIOS settings, and also when I checked from the Live USB which mode is in use, I got legacy: there was no /sys/firmware/efi device.
I tried a lot of solutions found online, and at the end I was able to install 18.04 on the disk when I created a MBR partition table.
I didn't keep track of all tests I performed, but I am quite sure I also tried to install 21.10 AFTER putting a MBR partition table, so I'm confused.
So, how does the Ubuntu installer pick UEFI vs legacy when defining the way to install the system?
I would like to install 22.04 as soon as it is released and I would like to reformat using ZFS, but I need to know that it will work in Legacy mode, or I need to know how to make it work in Legacy mode.
There is another similar question (How to avoid UEFI? Installation without UEFI? How to have Ubuntu 19, 20, 21, 22 w/o UEFI? Just no more EFI/UEFI problems?) which is however about how to force Legacy. The solution proposed (MinimalCD) is outdated, from what I understand from the Ubuntu Wiki, deleting the EFI partition from the USB stick (I think) didn't work in my case, and also I'm mainly asking about the way Ubuntu picks the mode.
Edit
To be clear: in the BIOS the (U)EFI has never been active, the live media booted in Legacy, and yet the new install didn't start, showing the "no OS found" error. I tried to activate UEFI in the BIOS and the message did not show up anymore (just got a blank screen), leading me to think the install was in UEFI mode (also due to the UEFI partition).
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