I have quite a few RB760iGS "hEX S" used as Fiber CPE/Routers. Last night, while updating one it failed to come up. I pulled a replacement off the shelf and upgraded it and it also stopped booting.
I was able to boot them via Netinstall/Netboot and they "Format Hard Drive" then during the Netinstall transfer they fail and stop responding.
I tried to Netinstall v7.1.3, v6.49.6 and v 6.48.6 but couldn't get a successful transfer. Eventually they stopped beeping at startup, and while the SFP light sequences to indicate NetBoot (Solid > Blinking > Solid > Off) they no longer show up in NetInstall.
I have tried powering with both PoE (24v Passive) and the included wall wart.