For NV, NV+, 1100, and Rev B X6/600 Power LED off, 3rd disk LED on: Unrecoverable NAND read error(Multi-bit error can not be recovered by ECC)
For Rev A X6/600/1000S Power LED fast blink, 3rd disk LED on: Can not find boot device Compact Flash
Power LED shall stay solid during normal operation or blinking/pulsing at regular rate during boot time, if it goes off about every 30 seconds, and activity LED blinks once when power LED goes off, system is stuck at disk retry.
If all LEDs blink briefly(milliseconds) after power on and then no sign of power, the PSU may be bad or there might be short circuit on PCB and hard drives.