Upgraded to Mavericks, now computer powers itself off on boot.
It this a known problem? On boot I get the gray screen with the apple and the spinner, combined with a flat progress bar on the bottom.
The progress bar gets about 25% of the way done, and then the machine powers itself off. Cmd-R does not work to put the computer into recovery mode, but target mode seems to work, and I'll get my data off the computer that way.
Computer is a 2008 Mac Pro with a 2-disk raid array.