My Ubuntu OS only works on my machine if I enabled Legacy mode and disabled secure boot. And I never boot into grub. I just want to ask how to boot into grub first since I am planning to dual-boot it with another OS in Legacy mode, and also because UEFI mode renders my machine unusable as it indicates that there are no bootable devices.
On a stock Ubuntu installation, you are using GRUB 2, even if you don't realize it. In some cases (like an Ubuntu-only installation), the GRUB 2 menu might not appear, but GRUB 2 does run. Without either GRUB 2 or some other boot loader, the Linux kernel can't load, so Ubuntu won't run.
(Note the "some other boot loader" caveat. LILO, GRUB Legacy, SYSLINUX, ELILO, and the EFI stub loader are all alternative boot loaders that will do GRUB's job [more or less]. None of these is installed and configured as the default boot loader, though.)
If you install Windows after installing Ubuntu, the Windows boot loader will take over the boot process, and will probably boot straight into Windows. To restore GRUB 2, you'll need to use Boot Repair from an emergency disk or re-install GRUB 2 (or some other boot loader) manually.
Note also that, on a modern computer, using BIOS/CSM/legacy mode is doing it the hard way. See this page of mine for details on why this is the case. Chances are you installed in BIOS mode because you prepared your boot medium incorrectly, although it could be some other problem that sent you down this path. In any event, you have two choices:
- Convert to an EFI-mode install by converting your disk from MBR to GPT form (optional, but strongly recommended) with
gdisk(see here for details) and installing an EFI-mode boot loader. You can then disable your CSM and install Windows in EFI mode.
- Install Windows in BIOS mode. Many older tutorials describe how to do this, and how to restore GRUB 2 once you're done, but I don't happen to have a URL handy. Be aware that you might accidentally boot the Windows installer in EFI mode, in which case it will likely complain about the MBR partition table on your disk (assuming that's what you've got). If you run into this error, you can either convert Ubuntu to boot in EFI mode, as in the preceding bullet point, or learn to control the boot process to get the Windows installer to boot in BIOS mode.