Wenn es gar nicht tun mag:
Legacy booten, beim Partitionieren dann manuell eine EFI-Partition mit 200MB (FAT32 mit Flags boot,esp) erzeugen und nach dem ersten LegacyBoot von der HDD dann halt das EFI-Subsystem ggf nachinstallieren. Da wird dann auch der EFI-Eintrag im BIOS gemacht. Über diesen Eintrag kann man das System dann sauber via EFI booten
Hier dazu eine wirklich tolle Doku: https://wiki.archlinux.org/index.php/GRUB
Syntax zum Nachinstallieren von EFI-Grub z.B.
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Installation
Note:
UEFI firmware are not implemented consistently across manufacturers. The procedure described below is intended to work on a wide range of UEFI systems but those experiencing problems despite applying this method are encouraged to share detailed information, and if possible the turnarounds found, for their hardware-specific case. A GRUB/EFI examples article has been provided for such cases.
The section assumes you are installing GRUB for x86_64 systems. For IA32 (32-bit) EFI systems (not to be confused with 32-bit CPUs), replace x86_64-efi with i386-efi where appropriate.
First, install the packages grub and efibootmgr: GRUB is the bootloader while efibootmgr creates bootable .efi stub entries used by the GRUB installation script.
Then follow the below steps to install GRUB:
Mount the EFI System Partition to either /boot or /boot/efi and in the remainder of this section, substitute esp with that mount point.
Choose a bootloader identifier, here named GRUB. A directory of that name will be created to store the EFI binary bootloader in the ESP and this is the name that will appear in the EFI boot menu to identify the GRUB boot entry.
Execute the following command to install the GRUB UEFI application grubx64.efi to esp/EFI/GRUB/ and install its modules to /boot/grub/x86_64-efi/.
grub-install --target=x86_64-efi --efi-directory=esp --bootloader-id=GRUB
After the above install completed the main GRUB directory is located at /boot/grub/. Note that grub-install also tries to create an entry in the firmware boot manager, named GRUB in the above example.
Remember to #Generate the main configuration file after finalizing #Configuration.
Tip: If you use the option --removable then GRUB will be installed to esp/EFI/BOOT/BOOTX64.EFI and you will have the additional ability of being able to boot from the drive in case EFI variables are reset or you move the drive to another computer. Usually you can do this by selecting the drive itself similar to how you would using BIOS. If dual booting with Windows, be aware Windows usually has a BOOT folder inside the EFI folder of the EFI partition, but its only purpose is to recreate the EFI boot option for Windows.
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Das sollte in jedem Fall dann sauber funktionieren. Habe ich oft schon nachträglich bei älteren Installationen gemacht, um sie auf neuer Hardware mit EFI sauber zum Laufen zu bekommen
Ein sehr gutes Tool, um auch missglückte Bootlader-Installationen wieder starten zu können ist ein “refind-Bootstick”
Der bootet im Prinzip alles, was er auf der HDD als startfähig erkennt…