Allied Telesis x600-48Ts/XP - Manual (Page 82)

Chapter 5: Troubleshooting
82
System Fault LED is Blinking
A blinking FAULT LED indicates that the switch is updating the active boot
configuration file or a new version of the operating system software is in
the process of being downloaded to the switch. The LED stops blinking
after the switch has completed updating the boot configuration file or
downloading the operating system software.