I’m on Win10, but for what it’s worth, I had the exact same issue, and it was a missing / bad driver. Searching also turned up bad USB cable or USB 3.0 port as possible issues.
I had the SOS + 1 blink on photon immediately upon powering and when trying to enter safe mode. Was able to enter DFU mode, flashing yellow, however dfu-util couldn’t find the device. In Windows device manager, saw “Photon DFU Mode” was missing a driver (although previously installed). I installed following Tutorial: Installing DFU-Driver on Windows [24 Feb 2015], I was then able to run particle update, and the Photon now behaves.