[Windows 10 Mobile] TristanLeBoss:
My answer to your PM may be useful for everyone. So I copy it here:

I'm not sure flashing the FFU of the Lumia 950 XL to an Ativ SE will work as is. Indeed they don't use the same SoC. It's possible to install the CAB files (HAL, ...) for the Qualcomm MSM8974AA v2 SoC using ImageApp.exe (from Windows 10) directly on the FFU file:

https://msdn.microsoft.com/en-us/library/windows/hardware/dn789232(v=vs.85).aspx

TO TEST

It may also be possible to install the CAB files (HAL, ...) for the Qualcomm MSM8974AA v2 SoC using DISM (from Windows 10) directly on the FFU file:

Use the /IgnoreCheck argument if you want the command to process without checking the applicability of each package.

You may want to remove the CAB files related to the Qualcomm MSM8994 SoC of the Lumia 950 XL:

Use the /Get-Packages option to find the name of the package in the image.

END - TO TEST

Without this step, you will probably ends up with a boot looping phone but you should be able to recover the original Windows Phone 8 OS with the Microsoft Windows Device Recovery Tool.

There is 4 ways to flash a new image to a device:

- Using UEFI download mode (Windows Phone)

- Using fastboot (Android)

- Using the Qualcomm 9008 mode (all phones with a Qualcomm SoC)

- Using the FFU download mode (Windows Phone)

1. How to put the phone in UEFI download mode

If you have a Windows Phone, please try to turn off the phone and then press the power button for a long time until the phone shows a "Windows Phone Boot Menu" with a "USB Mass Storage Mode". You can then release the button.

To select this option, simply press the power button once: you can now connect the phone to your computer. The driver should install automatically and the phone will appear on your computer as a disk drive.

2. How to put the phone in fastboot mode

Go here: http://www.droidviews.com/how-to-bo...astboot-download-bootloader-or-recovery-mode/

3. How to (force) trigger the Qualcomm 9008 mode

NOTE: If you have a working operating system, this is not needed to flash a new image. If the phone feels the need to go in Qualcomm 9008 mode (if it fails to boot from the eMMC), it will do it by itself.

NOTE: For Android, you may be able to reboot the phone in Qualcomm 9008 mode using an adb command:

The only way to (force) go into Qualcomm 9008 mode is to brick your phone: indeed, if it fails to do any kind of software boot from the eMMC memory, it will fallback to Qualcomm 9008 mode.

( Ok, on some phone, shorting some pins on the motherboard may force the Qualcomm 9008 mode but you need to know which ones :D )

The first step is to prevent the phone from booting the installed operating system. To do that, we remove the boot partition from the eMMC. Don't worry, the primary boot loader (PBL) is in a chip on your board and can't be erased: you only kill the operating system (Android/Windows) bootloader; also known as secondary boot loader (SBL).

3.1 Android

If you have an Android phone, you will need to boot into fastboot and issue the following command to erase the SBL1 partition (the operating system bootloader):

3.2 Windows

If you have a Windows phone, you will need to boot into UEFI download mode.

Once the phone is installed on your computer, you need to delete the SBL1 partition from the phone (the operating system bootloader). I think you can do it using any partition software because the phone is detected by the computer as a drive.

You can also use the emmcdl.exe tool:

You first need to list disks:

Note the number at the begining of your phone's drive. Then issue the following command (replace X with the disk number):

4. How to flash a new image

4.1. Using UEFI download mode (Windows Phone)

NOTE: If you don't have the stock FFU file or if your phone is not supported by the Windows Device Recovery Tool, you may also want to do a backup of the phone drive using a free tool like "HDD Raw Copy": this drive image can be reflashed later using the download mode [if you can reach it] or from the Qualcomm 9008 mode.

4.1.1 Method #1: FFU > VHD > HDD Raw Copy

- convert the FFU file to a VHD using ImgMount ( http://xdaforums.com/showthread.php?t=2066903 ),

- mount the VHD as read-only on your PC using VHDAttach ( https://www.medo64.com/vhdattach/ ),

- use "HDD Raw Copy" ( http://hddguru.com/software/HDD-Raw-Copy-Tool/ ) to copy the whole disk corresponding to the VHD to the drive corresponding to your phone (clone the VHD onto the drive).

4.1.2 Method #2: Using DISM.exe

NOTE: You will probably need DISM from a Win 10 installation.

Use /SkipPlatformCheck if the FFU file being applied is targeted for a device other than the device performing the application.

URL: https://developer.microsoft.com/en-us/windows/iot/win10/samples/dism

4.2 Using Qualcomm 9008 mode (All phones with a Qualcomm SoC)

If your phone is in Qualcomm 9008, it will show up as "Qualcomm HS-USB QDLoader 9008" [or QHSUSB_DLOAD]. (If it does show up as "Qualcomm HS-USB Diagnostics 9006" [or QHSUSB_BULK] don't do anything as it's not exactly the same thing).

You indeed need drivers. You can find them here: https://mega.nz/#!uhJl0B5R!J6Hbx6Dd6...WrkVNK8IZaOTkg

NOTE: It seems that some phones also mount the eMMC as a drive: you need to look under "Disk drives" in your Windows "Device manager. Indeed, if you find "Qualcomm MMC Storage USB Device", it means your phone eMMC is mounted as a drive. You can use methods from 4.1 instead of continuing here.

Once the phone is detected and installed, you can use this command:

Replace COMX with the port used by the phone: you can find it in the "Device Manager" under "Ports (COM & LPT)". You also need the correct flasher for your phone.

// TODO : Add things about protocol

4.3 Using fastboot (Android)

Extract all partitions from the FFU file as BIN files:

Flash new partition table:

Flash all partitions:

Reboot:

4.4 Using FFU download mode (Windows)

NOTE: This method use a flasher from the phone. This flasher does check for image integrity (thanks to the embedded hash table into FFU file) and check if the FFU file is signed (thanks to the embedded catalog file). It also check if the FFU file is for this phone (platform check). So, it can only be used to flash stock image.

To force the device into the FFU download mode manually, press and release the power button to boot the device, and then immediately press and hold the volume up button. This option is available only after an initial FFU has been flashed to the device.