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for my work I need some application on Windows7 and Ubuntu16.04 LTS. That'll lead me into a dual boot situation which I need.

I've done the space allocation on Win7 like described within The Dell Support Site as well as the BIOS Settings with Secureboot and switched to UEFI.

After Reboot I can choose from the UEFI Bootmenu (rapidly tapping F12 on the host) to boot Ubuntu. I clicked on try Ubuntu and wanted to install as described in The Dell Support site.

When go through the installation a popup asks me whether I want to Force UEFI installation. I'm not quite shure if I want that to because it says "this machine's firmware has started the installer in uefi mode bit it looks like there may be existinf operation systems already installed using 'BIOS compatibility mode'. If you continue to install Debian in UEFI mode, it might be difficult to reboot[...]"

The question might be similar to this one but the only solution is to use rEFInd. Is there no other way to install both on the same system?

The PC came preinstalled with Win7, I did not installed it myself.

Thanks for the help!

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Overall, I agree with duncan, but I want to elaborate on his comment.

Old computers (most x86/x86-64 systems from before mid-2011) booted using a firmware type called the Basic Input/Output System (BIOS), which usually booted from disks that used the Master Boot Record (MBR) partitioning system. Both of these technologies were created in the 1980s, so they're very old by computing standards, and they've been showing their age for quite a while.

The successor to BIOS is a firmware type called the Extensible Firmware Interface (EFI) or its version-2.x variant, the Unified EFI (UEFI). Computers that use an EFI/UEFI generally boot from disks that use the GUID Partition Table (GPT). Confusingly, many people, and even manufacturers, refer to their EFIs as BIOSes, or sometimes as "UEFI BIOSes" or something similar. My guess is that the manufacturers do this because people know more-or-less what a BIOS is and substituting the term EFI or UEFI would be confusing; but this mis-terming creates its own confusion because people drag their BIOS knowledge along and try to apply it to EFIs, which leads people to make mistakes. The bottom line is that BIOSes and EFIs are very different types of firmware to tackle the same task of beginning the boot process.

Fundamentally, BIOS and EFI are incompatible boot systems -- a BIOS-based computer can't boot with an EFI boot loader and vice-versa. To ease the transition, though, most x86-64 (aka AMD64 or x64) EFI-based computers ship with a Compatibility Support Module (CSM), which enables them to boot with a boot loader designed for BIOS. The CSM is something like dosemu in Linux, in that it allows a newer environment (EFI or Linux) to run programs for an older environment (BIOS or DOS). One limitation of the CSM is that switching between BIOS-mode and EFI-mode booting is usually awkward. This awkwardness is largely a matter of user interfaces -- the switching requires use of the computer's built-in boot manager, which on most computers is very primitive. Another limitation is that some OSes, including Windows, tie the boot mode to the partition table type -- Windows will only boot in BIOS mode from MBR disks, and booting in EFI mode works best from GPT disks. This second factor makes it very difficult to install one OS in BIOS mode and another in EFI mode. To be sure, there are ways to do it, but they're awkward and seldom worth the effort.

Most computers that shipped with Windows 7 used plain BIOSes, but toward the end of the Windows 7 era, EFIs became available, so some Windows 7 systems shipped with EFIs. Some of these booted natively in EFI mode, but others relied on their CSMs to boot in BIOS/CSM/legacy mode. It sounds like your computer fit in the latter category. As such, you have three choices when setting up a dual-boot system with your existing Windows installation and a new Ubuntu installation:

  • Boot both in BIOS/CSM/legacy mode -- This is the simplest thing to do; however, if you accidentally boot the Ubuntu installer in EFI mode, you'll get the warning you describe; and if you force it to install in EFI mode, you'll end up with a mixed-mode installation at best and an Ubuntu that's unbootable at worst. To force a BIOS-mode installation, you must learn to control your boot process, either by selecting the BIOS-mode boot option for the Ubuntu installation medium or by removing the EFI-mode boot loader from the Ubuntu installation medium. Unfortunately, details of controlling the firmware vary from one computer to another.
  • Boot both in EFI/UEFI mode -- This is a possibility, and in some ways it's preferable to the preceding one; however, it will require you to convert your Windows installation from a BIOS-mode to an EFI-mode boot. This conversion is possible, but it's tedious, and there's a significant risk that it will go badly. "Go badly," in this context, means that your computer will no longer boot Windows and you'll need to re-install it! If you want to pursue this option, see this blog post for instructions. Microsoft also has a new tool, called MBR2GPT.EXE, that's supposed to be able to do the job; but it's only available with the latest Windows 10 version.
  • Do a mixed-mode installation -- If you tell the installer to continue at the prompt you've described, this is what you'll get; but depending on how the firmware is set up and how you partition the disk, the boot loader might not work, or even be installed at all. The easiest way to do this type of setup, given your starting point, is to create a ~550 MiB FAT32 partition with a type code of 0xEF in addition to the regular Ubuntu partitions. This 0xEF partition is an EFI System Partition (ESP), which holds EFI boot loaders. You can then install my rEFInd boot manager to the ESP and edit its refind.conf file: Uncomment the scanfor line and ensure that it includes hdbios as one of its options. When rEFInd launches, it will probably include an option to boot Windows in BIOS mode; and it will also provide at least one option to boot Ubuntu. Note also that mixed-mode setups are impossible on some (rare) computers. AFAIK, no Dells are among these.

Clearly, the easiest solution is to do a BIOS-mode Ubuntu install. The main difference between this approach on your EFI-based computer and the same thing on an older BIOS-only computer is that you'll need to learn more about controlling the boot process, so as to avoid accidentally installing in EFI mode. Unfortunately, details vary from one computer to another.

You may want to read my page on the CSM for some more background information. Be aware, though, that I wrote that page for the benefit of people with Windows 8 or later systems that currently boot in EFI mode and who might be instructed or tempted to install Linux in BIOS mode, not the other way around. Thus, the emphasis of my page is on preventing inappropriate use of the CSM, whereas you probably want to guarantee use of the CSM for your Ubuntu installation.

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