Q When I tried to wake my early 2008 Mac Pro up, it had apparently tried unsuccessfully to restart, leaving the Power light flashing. Why?
A Flashing of the Power status light is your Mac’s way of reporting a hardware error detected during power-on self-test (POST) routines.
The flashing light codes are explained here: when the light flashes about once a second, it indicates that it has not found any RAM, or that found is damaged or out of specification. If flashes occur in groups of three, some RAM found cannot be used by OS X.
In either case you should shut your Mac down and turn the mains supply off, open its case to gain access to the installed RAM, then clean and reseat the modules before closing the case and trying again.
If it still reports a RAM fault, repeat that process removing any third-party RAM modules to see if they are at fault.
Once you have identified which modules are causing the problem, you will need to replace them, in your case with 240-pin PC2-6400 (800MHz) DDR2 ECC fully-buffered DIMM (FB-DIMM) modules installed in matched pairs.
Comments Other Intel Macs use a system of startup tones to signal a wider range of problems. Apple details those here.
Updated from the original, which was first published in MacUser volume 29 issue 09, 2013.