Stuck in a boot loop after failed update to Mavericks, can't reformat HDD
I’m afraid your hard drive is dying. Updating can be somehow hard drive intensive, so it’s not that surprising that it died during an update. If you have no backup, copy your important data to another hard drive with another computer1.

You can still try to save it with utilities such as DiskWarrior.

Otherwise, you’ll have to buy a new hard drive and install it in your Mac Pro, then copy your data back to it.




And start doing regular back up right now.