OK so you can’t boot from a USB on Surface Pro 3 using Windows 10 because the login screen won’t load and it just keeps flashing. You tried the volume down idea but that didn’t work and you are going mad? Well HOPEFULLY this will help. I had the same problem but fixed the issue with a few simple steps as follows (note I can’t guarantee this will work for you but I was ready to give up):
1) Don’t try the volume down button method, it may work for some but I could never get it working.
2) Go out to CVS/Walgreens and buy an $8 Flash Drive. Yes they are that cheap now and buying a new one removes the potential for there being some issue with any you happen to have lying around.
3) Visit this page: http://tinyurl.com/qfnyqbb and follow the following sections to the letter, don’t skip anything or rush anything (as that’s the initial mistake I made):
“Set up a USB device as a bootable drive”
“Configure your Surface to start from a USB device”
4) Once you have completed both those steps, with your new Flash drive inserted, go to this page and follow the steps to the letter: https://www.microsoft.com/en-us/software-download/windows10 Again don’t rush. Read slowly and carefully.
5) Now go BACK to this page: http://tinyurl.com/qfnyqbb and follow the section titled (You simply need to stick the USB Flash into the USB port (while shut down) and reboot. The reboot will magically take you to a Windows install screen:
“Surface 3 (and Surface Pro 3 with a configured boot order in UEFI)”
6) Don’t worry you don’t have to install a new version of windows and lose your files; if you click the install option you will be given some options one of which should be “Troubleshoot”. There you should get the option to recover your Windows installation WITH files kept intact.
Important note: Just follow onscreen instructions from there but note that the Surface will install new software and at some point appear to go back to the start, asking you to download Windows 10 again; the only reason it did this was because the system tried to restart but you still had the USB plugged in. Remove the USB Flash at this point and reboot and Windows will begin installing further until you ultimately have to a fresh, recovered Surface Pro 3.
Important note 2: While all my files remained intact, the recovery did remove my applications like Firefox and Chrome etc. So you will have to spend a little time reinstalling those.