Hi there Andreas Mellis,
That is unpleasant.
This could be a connection related issue. So, one simple thing you can start with is to just attach your internal drive with different power and SATA cables to another SATA port.
In case the issue persists, it may be a good idea to back up the crucial data stored on it. Just back up the most important stuff in small portions.
Then, you can check the drive's health out with some of these:
http://www.tomshardware.co.uk/forum/282651-32-best-diagnostic-testing-utility
You can even test both your drives.
If the drive turns out to be in a healthy state and you've already attached it with different cables, then you can go to your MOBO's website and update the SATA drivers.
If there is something wrong with the drive, then you will need to back up all the data and replace.(RMA if under warranty)
Let me know how this goes,
D_Know_WD