could be any number of things. It could be that the drive is full (never fill a HDD more than 85-90%... Windows freaks out). It could be that the HDD is going to sleep too early and needs to spin-up before playing back your media. It could be a virus or malware that is causing you some issues. Or it could simply be that the drive is dying and needs to be replaced.
We would need to know more info to be able to say what is or is not wrong with it.