Installing Windows 7 Ultimate on a Dell XPS 8900 that originally had Windows 10

JL Nevarez,

Below are some links to hopefull help your Windows 7 installed on your computer.

The default boot mode in the BIOS is set to UEFI and the operating system is installed in the UEFI Mode from the factory. You will need to change the Boot Mode in the System Setup from Legacy to UEFI Mode. 

How to Downgrade Preinstalled Windows 10 to Windows 7/8.1

How to manually reinstall Windows 7 on a Dell computer?

xps 8900 Drivers, Downloads and Manuals

Drivers Help, Tutorials and Install Order

While trying to install Windows 8 or Windows 7 on a computer with a single 3TB HDD, the windows installation will detect only 746 GB (No specific reason for this magical number. It could possibly be because of the MBR partition table allocation). Same way if the system on RAID 1 or 0 and there are 2 HDD, the system will show only 1468 GB.

Description:
This issue happens due to conflict between UEFI, MBR and the way the partition table is structured. Even if the MBR structure is converted to GPT, on the Windows Installation screen, you will see an error - "Windows cannot be installed to Disk 0 Partition 3 (show details)" - clicking on "show details" gives error "Windows cannot be installed to this disk. The selected disk is of the GPT partition style."

Solution:
Follow the steps below to resolve the issue:

Restart the system and go to the BIOS. Reset the BIOS defaults and restart the system.
Boot to BIOS again and change:
The SATA Operation to RAID.
The Boot Mode to UEFI.
Enable Secure Boot if required.
If RAID setup is required - Restart the system and enter the Ctrl+I IRST BIOS. Set up RAID, At this point, the drive should be seen as their actual size (approximately 2.7TB using the 3TB drives).
Restart the system and boot to the F12 menu. Boot from the Windows CD-ROM or USB drive in UEFI mode (select the UEFI option for your drive from the boot list menu - there will be two options, one with and one without UEFI). It's important that you boot from UEFI mode, as this is the only mode that can create GPT drives.
Choose Custom Install (again, losing all data).
When you reach the screen displaying your disk drives, you'll notice they're showing the wrong size (746GB for single HDD or IRST configs/ 1468GB for RAID configs).
Click the Load Drivers button. Navigate to the place on the USB drive where you have the Intel RST drivers and click OK.
Click Next to load the driver. After loading the driver, your RAID1 array should show as one large drive of 2.7 TB.
Convert from MBR to GPT

From the screen asking which drive you want to install windows on, Press Shift + F10 to bring up the command prompt.
Type diskpart to open the disk partition utility.
Type list disk to list all of the drives.
Find the drive that is listed as the large drives and find what number it is listed as.
Type select disk # (where # is the number of your large volume, e.g. select disk 0 for example).
Type clean. This will erase all data about the drive and ensure it is as good as a raw drive.
Type convert gpt to convert the disk to a GPT disk from an MBR disk.
Type exit to exit diskpart and exit again to exit the command prompt.

A NOTE indicates important information that helps you make better use of your computer. NOTE:
If the computer restarted due to any reason of if you want to check to make sure the GPT conversion worked

Enter windows setup again, load the Intel drivers again, and bring up the command prompt by using Shift + F10 again.
Type list disk. This time, check to make sure that the disk you converted has an asterisk (*) in the GPT column.

Install Windows

Your large drive should now show as unallocated space.
Install Windows to the large RAID drive's primary partition as you normally would.
Boot into Windows and Extend the Drive

When Windows first boots, it won't recognize the drive as a full 3 TB drive -- 760 GB will be left unallocated.

From the start menu, open Computer Management (shortcut: type compmgmt.msc and hit enter)
Go to Disk Management.
Right-click your large drive structure and select Extend. Extend to fill the rest of the space.
You now have a 3TB RAID array fully recognized in Windows 64-bit. If you did not choose to install a RAID volume, you can now continue to install Intel RST Software and/or Intel management center for Acceleration.

OS installation on systems with M.2 SSD module.
When reinstalling the OS on the XPS™ 8900  follow these steps to ensure that they can install the OS without any issue:

Ensure SATA Mode is set to RAID or in some systems Intel Smart Response Technology in the setup.
Tap Ctrl+I to enter the RAID BIOS and disable the accelerate option for the SSD.
Initiate the OS reinstall. Follow it up by installing all the drivers including the Intel Rapid Storage Software.
Run the Intel Rapid Storage software through all programs or the taskbar icon.
Click Enable acceleration under the Status or Accelerate menu.

Select the SSD to be used as a cache device.
Select the size from the SSD to be allocated for the cache memory.
A NOTE indicates important information that helps you make better use of your computer. NOTE:

Any remaining space on the SSD may be used for data storage using the simple data single-disk RAID 0 volume that is automatically created.

Select the drive (or RAID volume) to be accelerated. It is highly recommended to accelerate the system volume or system disk for maximum performance.
Select the acceleration mode. By default, Enhanced mode is selected.
A NOTE indicates important information that helps you make better use of your computer. NOTE:

Enhanced mode: Acceleration is optimized for data protection.
Maximized mode: Acceleration is optimized for input/output performance.

Enable Acceleration 

Click OK. The page refreshes and reports the new acceleration configuration in the Acceleration View. Your system is now successfully configured with the Intel Smart Response Technology.