Dual boot with Windows 10 on separate hard drives

Question

So I have two internal hard drives. My original has Windows 10 (previously 8/8.1), and I installed Ubuntu 15.04 on my second one (after upgrading to 10). It installed fine and I can run it, but here's the problem: I can only to launch Ubuntu through UEFI mode. Through the boot from device option. I have disabled secure boot and quick boot through my BIOS and I have tried using EasyBSD, but neither seem to work. I would very much like to have the option to boot into either upon start up, is there something I am missing?

Answer 1

Switching boot modes (BIOS vs. EFI) is a bit awkward with most computers, and there's seldom a real need to do is. When there is such a need, it's to support OSes that require different boot modes -- for instance, OS X (EFI) and Windows XP (BIOS) on a Mac. Macs can handle this a bit more seamlessly than PCs, but they typically require an ugly and dangerous hybrid MBR to do the job. That's going off on a side track, though....

In terms of Ubuntu, the only reasons I can think of for booting in both BIOS and EFI modes have to do with developers working on boot issues and individuals wanting to learn about the boot modes. In both cases, IMHO using installations in virtual machines is likely to be a better option, since the installations will be closer to the relevant standard setups.

If you really must do it on a real machine, you'll need:

• At least one disk partitioned with the GUID Partition Table (GPT).
  • If you have two disks, the second can use the older Master Boot Record (MBR) system.
  • MBR can be used for EFI-mode booting, but some EFIs react badly to it and the Ubuntu installer will (IIRC) refuse to install in EFI mode to an MBR disk, so as a practical matter MBR is a bad choice for EFI-mode booting. GPT, OTOH, can be used in both modes, although it sometimes causes problems in BIOS mode.
• Assuming a 1-disk GPT setup and GRUB as the BIOS-mode boot loader, the GPT disk must contain a ~1MiB BIOS Boot Partition to handle BIOS-mode booting.
• The disk must have an EFI System Partition (ESP) to hold the EFI boot loader.

You can install in either BIOS mode or EFI mode, which will install the version of GRUB for that boot mode. After installation, you'll need to install the boot loader for the other boot mode. Doing this using APT will be tricky at best, since the EFI-mode and BIOS-mode GRUB packages are mutually exclusive; installing one will remove the other. Thus, you'll need to install one package in some way other than APT.

Alternatively, you can move away from GRUB for one boot loader. One particularly good choice is my rEFInd boot manager. rEFInd is an EFI-mode boot manager that has the ability to shift from EFI-mode to BIOS-mode booting. That feature is disabled by default, but can be enabled by uncommenting the scanfor line in refind.conf and ensuring that hdbios is among the options. When so configured, rEFInd will enable directly booting Linux kernels in EFI mode (without using GRUB) or switching to BIOS mode to boot via your BIOS-mode GRUB (or other BIOS-mode boot loader). Note, however, that this procedure doesn't work on all computers; some lack the necessary support.

If you don't use rEFInd, you'll have to use your computer's built-in boot manager to switch boot modes. This can often be done by hitting Esc, Enter, or a function key at boot time, but details vary greatly from one computer to another. Using this feature will require both a BIOS-mode GRUB and an EFI-mode boot loader be installed.

Overall, setting everything up correctly is likely to be a pain, and the configuration will be more delicate than a more conventional single-mode boot. Using the built-in EFI boot manager can be a nuisance, but rEFInd can help with that a bit.