I'm not sure what part you're talking about, but the User Manual has a diagram. Unfortunately, the ifixit bots are marking my link as spam, so I can't post it here. If you google "asus chromebook c300 user manual" without the quotations, and choose the second result, it should open the manual. The diagram starts on page 12.
Try a recovery disk if you have one. Alternatively, you can make one yourself. Personally, I use a program called Hiren's Boot CD to do diagnostics when anyone is having trouble booting. You can actually make it work on a thumbdrive. I would show you a link but ifixit bots are not letting me for the time being. I am not all that familiar with Macs, but there should be a system diagnostic called S.M.A.R.T. that you can utilize to determine if your hard drive is going bad. I will tell you, though, that more than likely your hard drive is failing after ten years.
Download a temperature monitor for your laptop. If it is overheating, try blowing out the heatsink, usually located right where the fan blows out. Do you only notice this problem while running on battery?