Most of the time, when a client has a drive which won't boot and I can't fix it in the computer, I remove the bad drive from the machine and put it in a USB docking station. This removes the possibility that a malfunction in the computer itself is causing the problem and allows me to either copy the data to another drive if possible, or run one of several rescue or repair programs that I use. This weekend, however, I ran into something new.
I had stopped in to see a client whose laptop wouldn't boot. Following instructions they had found online they had been able to boot to the OS X rescue partition but hadn't progressed much further than that. I decided to boot into single user mode and run fsck. The drive took much longer than usual to boot this way, but despite reporting an i/o error, fsck did find one error and was able to fix it. I then typed exit, hoping OS X would launch successfully, but instead got I two more i/o errors. I decide to shut down at this point and bring the computer home.
Once I got the machine home I opened it up and put the drive in the docking station. At this point I was stymied: the drive didn't show up in Disk Utility, and none of my other tools could see it either. I began to think I'd used up the last time this drive would mount anywhere. Recalling that the drive had mounted when I booted it at the client's house, I decided to try placing it back in the laptop and boot into target mode. I'd never seen this work before when a docking station had failed, but thought I'd give it a try. To my pleasure and surprise, it worked. I was then able to use CopyCatX to make a good image of the drive. It took a while, but it worked. So - if your docking station fails to mount a drive, consider putting it in a computer and booting into Target Mode.