I had a similar issue with my phone. It could be one of two things.

First - Have you installed any other applications onto your phone? If so try removing them and see if that solves the issue. That helped with mine

Second - If that doesn't work I have heard of an issue in which the directories don't match up. What your PC says is say the F drive is not what the Media manager says in the F drive. You can find the solution by doing a search of the forums and typing in "Media Manager Help" you should be able to find the solution there.

I know it has been posted but I'm sorry I cannot give you the solution. I didn't need it for my issue. But if you do the search you will definately find the answer

Let me know if it works though