OK, now I've got the same problem. Photon was running happily 24/7 and hadn't been rebooted since last November. Two days ago it started blinking SOS, one second pause, one red flash (I guess you'd call it a "T"). Also, the D7 blue LED is stuck on.
I am unable to enter Safe Mode. I can get it to blink magenta but when I release the Setup button, it goes right back to SOS-T.
If I keep holding down the Setup button, it eventually goes to fast flashing yellow (DFU mode?). At that point I get the "USB connected" da-ding sound from the PC it's connected to. The blue D7 LED remains on.
I tried a f/w update and it claims to have worked:
Microsoft Windows XP Version 5.1.2600 Copyright 1985-2001 Microsoft Corp.
C:\Documents and Settings\User>particle update
Your device is ready for a system update.
This process should take about 30 seconds. Here goes!
! System firmware update successfully completed!
Your device should now restart automatically.
You may need to re-flash your application to the device.
C:\Documents and Settings\User>Microsoft Windows XP
But then it went right back to SOS-T. I cycled the power - no difference. I tried Safe Mode again and got the same behavior as before. I tried redoing the f/w update - same results. I tried the Particle app - claims no devices found. Any ideas? Any chance the Blue Norwegian maneuver would help? I have one Photon left that could power that trick.