The only thing I can think of to try is to zap the p-ram. restart holding down option-command-p-r thru at least a couple of restarts. If that doesn't help, I suspect you'll have to take it in for servicing. If it's out of warranty, you might just get an external drive. There not a lot of money nowadays.
Thanks for your reply Michael.
I did try option-command-p-r but I could not get it to change anything. Then, I thought I would check the warranty.
I am a lucky person, it is still under extended warranty until 18th of this month. So I have booked it in to our local Apple shop, next Tuesday.
I will post the outcome.