Target Disk Mode

If the drive has failed and won't mount at all, it probably won't work in Target Disk Mode. But you can try it. First, try booting the computer from the Mac OS X install disk, choose Disk Utility from the menu, and if Disk Utility can see the hard drive, run a disk repair and see if that will get the drive going. If the drive has suffered a hardware failure (if Disk Utility can't even see the drive at all, a hardware failure is quite likely), the only option for data recovery would be to send the drive to a recovery service. Such services are very expensive and provide no guarantees that they can recover any useful data, but in the event of a drive failure they're the only option. File recovery software will only work on drives that have software problems (usually the disk has to be able to at least mount), not hardware failures.
www.cleverfiles.com/help/data-recovery-target-disk-mode.html
The above should be handy enough.