over the 6+ yrs there have been issues with Mac Pro when left alone for 1-2 weeks.
There is no clear cause or cure. Having it on UPS is best. Unplugged for that time has been blamed but hey, mine is unplugged for weeks and months at times during these 6.75 yrs.
The methods for resetting SMC have changed. And it partially depends on WHICH Mac Pro model you even have.
Ideally everyone has a bootable copy (clone) of their system that they keep on shelf - off line and safe - at all and any given time.
Free space alone is not an indication of bad sectors, bad directory, electrical "event" or not.
Having a 900W UPS / 1500VA is as good as weakest link, not everyone puts modem and router and EJ45 through surge filter, the CyberPower SMART 1500VA works best I've had after using APC for over decade.
Check is system profile showing all the RAM or not? parity issue? other?
Are you on Snow Leopard? Mountain? use Disk Utility Repair Disk from another boot device.
Remove ALL cables and power and hit power on for 10 seconds and then connect only the kb/mouse and dispaly.
If your computer won't turn on, try each of these steps:
reset SMC For "Mac wont' turn on or boot" and related
- Make sure that the power cord is plugged into a working wall outlet, and that it's properly connected to the power adapter. (To check if the wall socket is working, plug in a lamp or other electrical device.)
- Make sure that the power adapter is properly connected to the power port on the side or back of the computer.
- Disconnect all accessories that are plugged into the computer, such as a printer, hub, or other mobile device.
- Reset the computer's PMU, SMC, or PRAM. Refer to the instructions for your model:
Note: It is possible that the SMC could encounter an issue that may cause unusual system behavior typically related to the symptoms described below. In some cases, resetting the SMC may be the only correct method to resolve the issue,
General purpose Mac troubleshooting guide:
Creating a temporary user to isolate user-specific problems:
Identifying resource hogs and other tips:
Starting the computer in "safe mode":
To identify potential hardware problems:
General Mac maintenance:
UPS = Un-Interruptible Power Source, a device with large-ish batteries that conditions the power being sent to your computer and keeps it alive for a few minutes if the power fails.
SMC = System Management Controller, the simple computer that handles Power ON/Off, Bus speed settings, fan speeds, recharging for notebooks, and similar items for the computer system. PMU = Power Management Unit, was the previous-generation of the same type of controller for the same items.
PRAM = Parameter RAM, a battery-backed store for parameters and panic reports.
APC = a Brand of UPS
TLA = Three Letter Acronyms, the things that are driving you crazy.