I've seen maybe two dozen of these. Boots to gear or somewhere abouts there and hangs or shuts down. (this one was shutting down due to fsck fail safety) Try to boot off another hard drive and the boot volume won't mount. Run diskwarrior and it won't even scan due to a "hardware malfunction". OK, that's about it for options.
But I thought, hey, it boots a little, what can I do in single user mode? enough, it appears. attached another hard drive and booted into single user. mounted the internal drive read/write, used mount to manually attach the external drive, and was able to ditto the /Users folder to the other drive.
An interesting twist, this one wouldn't let me make a folder in /Volumes, or anywhere else for that matter, as a mount point for the second drive, but /home was there and available (and empty) so I used that as the mount point.
Unfortunately diskutil is not available in single user mode so I wasn't able to look at disk info on the bad drive. (is there a way to load diskarbitration framework by hand?) But at least I could "du -sh" to see how much data I was dealing with.
