Dual boot Ubuntu 16.04 with preinstalled Windows 10 Dell G5 5587

Question

I am trying to install+dual boot Ubuntu 16.04(LTS) with pre installed Windows 10(home) in my Dell G5 5587. Windows is installed in SSD. I have 1TB HDD alongside the SSD.

I did the following:

Freed 100 GB from SSD and created 100 GB unallocated space.
Turned off Windows 10 Fast boot.
Turned off Secure boot feature from BIOS. 
Changed Secure boot mode to audit mode in BIOS.

msinfo32 is showing:

BIOS Mode           UEFI
Secure Boot State   Off

The BIOS is showing UEFI on, Secure Boot off, PTT on.

Approach 1 (MBR+UEFI/Legacy)

I burned Ubuntu using Rufus(MBR+UEFI/Legacy) to a pendrive. I got the pendrive in the one time boot menu(F12). When I try to install Ubuntu it showed the following warning:

"This machine's firmware has started the installer in UEFI mode but it
  looks like there may be existing operating systems already installed
  using "BIOS compatibility mode". If you continue to install Debian in
  UEFI mode, it might be difficult to reboot the machine into any
  BIOS-mode operating systems later.
  
  If you wish to install in UEFI mode and don't care about keeping the
  ability to boot one of the existing systems, you have the option to
  force that here. If you wish to keep the option to boot an existing
  operating system, you should choose NOT to force UEFI installation
  here.")

As I do not want to risk losing Windows, I aborted installing Ubuntu.

Approach 2 (GPT+UEFI)

If I burn the Ubuntu OS using Rufus(GPT+UEFI), I do not find the pendrive in one time boot menu(F12).

Question

How to properly install dual boot Ubuntu 16.04 LTS with pre installed Windows 10 in Dell G5 5587?

arsho · Answer

Steps to install Ubuntu 18.04 alongside pre installed Windows 10 as dual boot in NVMe SSD.

Hardware Information:

Dell G5 5587
Core i7 8750H
RAM 16 GB
NVMe SSD (Toshiba) 256 GB
HDD 1 TB
NVIDIA® GeForce® GTX 1060 6GB GDDR5 with Max-Q Design

Download Files

Download Ubuntu iso file from Ubuntu website.
Download Rufus from Rufus website.

Burn the ISO to a Pendrive

Insert a USB pendrive.
Backup the pendrive data if necessary.
Open Rufus.
Select Partition Scheme MBR.
Keep the other settings to default. E.g.: FAT.
Select the ISO and burn it to the connected pendrive.

Create Unallocated Space for Ubuntu

Open disk management in Windows. 
Shrink the volume where Windows is installed (NVMe SSD). The size depends on yourself. I have created 100 GB of unallocated space.

Turn off Fast startup from Windows 10

Dual boot does not work when Fast startup option is enabled in Windows 10.
Right-click the Start button.
Click Search.
Type Control Panel and hit Enter on your keyboard.
Click Power Options.
Click Choose what the power buttons do.
Click Change settings that are currently unavailable.
Uncheck Turn on fast startup (recommended).
Click Save changes.

Turn off secure boot from BIOS

Tap F2 key at the Dell logo screen to enter System Setup or BIOS.
On the left pane, click Boot Sequence.
Check that Secure Boot is set to Disabled. Within the BIOS go to Secure Boot > Secure Boot Enable, and set the checkbox to Disabled.
Change the Secure Boot Mode to audit mode.
Save settings and the machine will be restarted.

Enable AHCI for dual boot

With a preinstalled Windows SATA mode set to IDE or RAID in BIOS.
To install dual boot we need to change SATA mode to AHCI from BIOS.
Create the Windows 10 local account:

Go to Settings > Accounts.
Select Family & other users.
Tap Add someone else to this PC.
Select I don't have this person's sign-in information.
Select Add a user without a Microsoft account.
Enter a username, type the account's password twice, enter a clue and select Next.
Change the Account type of this newly created account to Administrator.
Login using this new user account.

Right-click the Windows Start Menu. Choose Command Prompt (Admin).
Type this command and press ENTER: bcdedit /set {current} safeboot minimal
Restart the computer and enter BIOS Setup. On Dell Inspiron it is F2.
Change the SATA Operation mode to AHCI from either IDE or RAID.
Save changes and exit Setup and Windows will automatically boot to Safe Mode.
Right-click the Windows Start Menu once more. Choose Command Prompt (Admin).
Type this command and press ENTER: bcdedit /deletevalue {current} safeboot
Reboot once more and Windows will automatically start with AHCI drivers enabled.

Install Ubuntu

After the OS burn, insert the pendrive and restart the machine.
Press F12 or F10 depending your machine.
Select USB/Removable media.
Select install Ubuntu.
Select Language and other options.
When it comes to partition option, select something else.
In the unallocated space:

Give 2 GB Logical space to swap memory. The swap size depends on your RAM size.
Give remaining Primary space to / partition

Select Windows Boot Manager as Device for boot loader installation.
Continue with the remaining process.

Install Drivers (optional)

After successful installation of Ubuntu, update the system softwares.
To update the drivers:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
ubuntu-drivers devices
sudo ubuntu-drivers autoinstall

You may now access both Windows and Ubuntu in a dual boot manner.

Reference:

Turn off fast startup Windows 10
Dual boot in NVMe SSD Dell
Turn off secure boot from Dell BIOS
Create local account in Windows 10
Dell documentation on creating dual boot