Mac Pro 2,1 Only Shows Flashing Question Mark After Update So, I have an older Mac Pro 2,1 that I received.  Had a fresh install of Mavericks and nothing else.  It worked fine, with exception to slow redraw speeds on the screen.  When I got it plugged into a network for the first time, it said there were updates, so I let it do the updates.

When I returned to the machine, it was simply sitting at a white screen.  After waiting several hours with no change, I turned the machine off, and rebooted.  I was met with the flashing question mark on a folder icon.

The Mac has the Apple RAID card in it, and 4 1TB drives.

Pressing any of the startup commands during boot does nothing; I still get the flashing icon.  CommandR doesn't work.  C doesn't work.  The only two that do anything is CommandOptionPR (reset NVRAM) and Option (Startup Disc select).  However, even when using those two, they still result in a flashing question mark icon.

Any clues?  I'm wondering if the RAID card went bad or something.  It was complaining of a bad battery.
A flashing question mark means it can't find system software, however, it is able to find the boot volume.  If the boot volume is on the RAID array, then it's not the RAID card.  

Try booting from installation media to see if you can see the drive.  If you can see the volume and mount it, you can recover your data.

As for the RAID battery, it keeps the last last set of write commands in the controllers memory in case there is a power failure.  I would replace it because you lose that safety net if the battery has no charge, but it's not the cause of your issue.