Unable to install ubuntu alongside windows 10 on separate disk

Question

I have a system with three drives, one ntfs boot ssd for windows 10, one ntfs hdd for storage, and another to install ubuntu on.

I am getting the

Force UEFI Installation? This machines's firmware has started this installer in UEFI mode but it looks like there maybe existing operating systems already installed using BIOS compatibility mode, If you continue to install Debian in UEFI mode,it might be difficult to reboot into any BIOS-mode operating system.

Message, so I tried both booting from the live usb in non uefi (doesn't boot at all, just goes straight to windows), and installing by forcing uefi - this installs successfully, but will not boot, even with the other drives disconnected.

The motherboard in use is an Asus P8Z77-V, and as many 'fast-boot' settings as I can find are turned off.

parted -l:

Model: ATA WDC WD10EZEX-00W (scsi)
Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos
Disk Flags: 

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  525MB   524MB   primary  ntfs         boot
 2      525MB   1000GB  1000GB  primary  ntfs


Model: ATA KINGSTON SUV400S (scsi)
Disk /dev/sdb: 120GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos
Disk Flags: 

Number  Start   End    Size   Type     File system  Flags
1      1049kB  120GB  120GB  primary  ntfs


Model: ATA WDC WD3200AAKS-0 (scsi)
Disk /dev/sdc: 320GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 

Number  Start   End    Size    File system     Name                  Flags
1      1049kB  538MB  537MB   fat32           EFI System Partition  boot, esp
2      538MB   312GB  311GB   ext4
3      312GB   320GB  8455MB  linux-swap(v1)

Answer 1

From your partition table, it looks like Windows is almost certainly installed in BIOS/CSM/legacy mode, but it appears you've installed Ubuntu in EFI/UEFI mode. This combination complicates booting, but can be made to work. There are four basic approaches you can take:

• Install a BIOS-mode Linux boot loader -- You can install the BIOS version of GRUB, LILO, or some other Linux-native boot loader. I don't have a pointer to tell you precisely how to do this, but it is possible, and once you configure the computer to boot exclusively in BIOS mode, it should work fine. Note that one way to do this is to re-install Ubuntu, but get the installer to start up in BIOS/CSM/legacy mode. This particular variant is overkill, but should get the job done, and is worth considering if you haven't yet invested much time in customizing your Ubuntu installation.
• Convert Windows to boot in EFI mode -- This task is likely to be very difficult and carries some risk of catastrophic data loss, so I don't recommend it for most people. If you're interested, see here for more information. Note that once you've converted Windows in this way, you may need to use EasyUEFI or bcdedit to re-enable GRUB as the default boot manager.
• Use the computer's built-in boot manager -- Most modern computers have a built-in boot manager that can switch between BIOS-mode and EFI-mode booting. You'll have to hit a particular key (usually a function key, Esc, or Enter) early in the boot process to get access to the boot manager. Once it's active, you should be able to select a BIOS/CSM/legacy option that will boot Windows or an EFI option (probably called "ubuntu") that will boot Ubuntu. This approach should work now, with no new software or reconfiguration required, but getting to the boot manager menu is awkward enough on many computers that this approach may be unappealing in the long run.
• Install rEFInd and tweak its configuration -- My rEFInd boot manager can switch between BIOS-mode and EFI-mode booting; however, the BIOS-mode boot options are disabled by default on PCs. To enable them, you must install rEFInd and then edit the /boot/efi/EFI/refind/refind.conf file: Uncomment the scanfor line and ensure that hdbios is among the options. With that done, the next time you reboot, you should see one or more options to boot Ubuntu and one to three BIOS-mode boot options, one of which should boot Windows. You may then want to use the dont_scan_* options to clean up the rEFInd boot menu to remove those that don't work or that are redundant. (If nothing else, chances are you'll need only one BIOS-mode option, but rEFInd may show one for each disk, so trimming the extras may be worthwhile.)

Overall, it's best to install all your OSes in the same boot mode to begin with. Unfortunately, controlling the boot mode for your installation medium is tricky even for experts, so problems like yours are common. See my page on the CSM for more on this issue. (That page is written with an assumption of Windows being in EFI mode and the perils of installing Ubuntu in BIOS mode rather than the other way around, but the principles are the same either way.)