It sounds like the hard drive is not working. Perhaps the circuitry is fried, and that's why it only appears sporadically.
You weren't very clear about the error messages that Windows gives you. What exactly did it say? Also, what is X51L?
I can't suggest much right now, but, if the drive turns out to be dead, you'll need to call the drive manufacturer (Toshiba, in your case).
I got X51L and my hdd stopped working , so I bought new one SATA: PM Toshiba MK1665GSX.Sometimes when i turn on my laptop, it doesnt detect my hard disk.. after few restarts it recognize the hard disk but when loading Windows it stoppes and says it needs repair.
First I tried with Windows 7, after the instalation it works but when I restart/shut down, it cant load again. The repair starts but can not find any errors and shuts it down.I thought that it has something to do with my OS, so I format and instaled XP.. Same happened.. It worked after the instalarion but when I shut it down and turn it off - in the begining of Windows loading it says "Windows could not start because the following file is missing or corrupt: \Windows\System32\Config\System.
Please help me , how to fix it ?