CGiles.You dont ned to replace the hardrive. The same thing happened to mine after 5 years of virtually zero problems. I opened up mine last week when i got the same error message. I disengaged the ribbon connector from the hardrive and then reconnected it. Literally just pull it from the hard drive and then push it back. then close the keyboard. If its a lose connection this should sort it out.

If that doesnt work you can check the hardrive by puchaseing an IDE HDD docking device for about 30 dollars.