Q:

How to install Ubuntu while keeping my files' backup disk partition I made with windows (D:)?

So I have my 2TB disk into two partitions (C:) and (D:), I don't care about (C:) which hosts windows. I want only Ubuntu as an operating system while keeping everything at the (D:) partition.
Any idea if that is possible?

A:

A) UEFI/BIOS

Set to "UEFI mode only" (no legacy/CSM).

Disable "secure boot"

Disable "Intel Rapid Start" (if equipped)

Disable "fast boot" in UEFI (note this is different than the "fast startup" setting in Windows 8/10). The options in your UEFI/BIOS might say something like Full/Minimal/Automatic for boot mode. Select Full (or thorough, or complete, etc whatever your UEFI vendor has chosen to call it).

B) Advanced Power Options (Fastboot)
Disable fast startup in Windows 8/10 under "advanced power options". Restart the computer to ensure that this subsequent boot and the next reboot/shutdown will be in "normal" mode.
C) Bitlocker (For Windows 10 Pro)
If you have Windows 10 Pro and encrypted the drive with "Bitlocker" remove the encryption.
D) Rufus / Bootable USB stick
Use Rufus to create a bootable USB stick with your choice of Ubuntu based distro. Make sure in Rufus that you CHOOSE the option UEFI/GPT only. This ensures the Linux environment boots only into UEFI mode during your install.
E) Boot Menu
Reboot your computer and press key for one time boot menu (Dell is typically F12). Select your USB stick from the boot options.

Note:
Make sure it says UEFI in front of the USB stick in the boot menu.
If not, return to Windows and recreate your USB stick with Rufus ensuring you choose the UEFI/GPT (only) option.

F) Boot into USB Stick
Boot into Linux live environment and begin the install.
G) Installation type
When you get to the installation option, choose "Something else" at the bottom of the Ubiquity installer.
H) Create partitions
Find your C: partition. It will be called something like /dev/sda1
Select it and press the "-" sign to delete it and create "free space".

1st Partition / Root (All the software you install are stored here)

Select "free space" that you created by deleting the C: drive.

Select "+"

Partition the target drive as follows:

Size: min. 10 GB (25GB+ recommended. I have 40GB)
Type for the new partition: Primary
Location for the new partition: Beginning of this space
Use as: ext4
Mount point: Choose "/"

2nd Partition / Swap (Only needed if you want to Hybernate)

Select "free space"

Select "+"

Partition the target drive as follows:

Size: Depends on your RAM. See Swap FAQ.
Type for the new partition: Primary
Location for the new partition: Beginning of this space
Use as: swap

4th Partition / Home (Only needed if you want to keep your personal files separate from / Root partition. It seems you save your personal files in D: so you may not need this.)

Select "free space"

Select "+"

Partition the target drive as follows:

Size: Remainder of "free space" or any size you want. (You will need to leave some space if you want to make another partition. Of course, you can always shrink "/home" partition later)

Type for the new partition: Primary
Location for the new partition: Beginning of this space
Use as: ext4
Mount point: Choose "/home"

I) Installation & Reboot

Finish the installation process and reboot (removing the USB stick when your UEFI/BIOS screen logo appears).

J) Upon reboot

Boot into Linux

Install any updates

**K) Setup Automout for D:

Open the "Disks" utility.

Select the partition you want to automount. (D:)

Click the icon with 2 cogwheels. (Additional partition options)

Select "Edit Mount Options".

Turn the "User Session Defaults" option to "Off".

Check the box "Mount at system startup" Make sure "Show in the user interface" is also checked.

Replace the line that says nosuid,nodev,nofail,x-gvfs-show to users,uid=1000,dmask=027,fmask=137,x-gvfs-show,utf8.

Change the "Mount point" to something less complicated like /mnt/D or /mnt/MyFiles.

Change the "Filesystem Type" to ntfs-3g.

Click "OK" and you are all set!

Borrowed and edited from user613363's answer. (Dual Boot Windows 10 and Linux Ubuntu on Separate Hard Drives)

