Have to add this here so people don't waste days trying to fix this(like I did) and can be found in a Google search.
Issue:
Sensei [RAW] firmware failure with orange blinking light. Steelseries Engine 3 will not recognize the mouse is present 99% of the time. Windows will not recognize the mouse at all.
Solution:
STEP 1 - Randomly plug the mouse into multiple USB ports, restarting the computer each time (you will waste hours and pull out a lot of hair) until Steelseries Engine 3 finally recognizes that the mouse is attached, it won't work but it will see it there in Engine 3 and attempt a firmware update (*which stalls at the start)
STEP 2 - When Engine 3 (finally) recognizes the mouse is attached it will ask you to perform a firmware update. This will not work, the progress bar will not progress no matter how long you wait. If you re-boot it is likely the mouse will not be recognized and you will have to go back to STEP 1 for a while until it recognizes again.
STEP 3 - Download and run SSEFix_1.27.0.0.exe, *IMPORTANT* This will only work if you have the STALLED Engine 3 firmware update window present (see attached image) otherwise you get a 'bootloader' error while trying to flash the mouse.
With the combination of recognized mouse by Engine 3 and stalled firmware update the SSEFix executable will work without the 'bootloader' error.
Please note I had another issue with both computers I was using, windows (8.1) would not boot into windows with the mouse attached, even disabling USB boot from BIOS, these mouse are pretty freaky!
I had to unplug the mouse, boot into windows startup then plug back in before login.
Image of firmware update working without bootloader error