Hi there compunovice,
I would advise you to back up the important data that is stored on the drive as soon as possible as it may be failing. After that, you can just test it with a brand specific(or a third party one) HDD testing tool. The results will show you the overall health status of the drive. If it has some bad sectors on it(or some other issue), you will eventually have to replace it.
Hope this will help,
D_Know_WD