Hey, It may just be that you have to reformat your harddrive.
First: Copy the files on the harddrive onto your mac desktop
Second: Go to Finder -> applications -> Utilities -> disk Utilities
3rd: select your hard drive on the left hand side list
Chose the Erase tab.
And for the volume format, chose MAC Os Extended
Let it reformat, when it's done, open up your hard drive, and copy back in the original files back onto it
You should be able to drag in files and what not!
I hope this helped, hopefully your problem is what I think it is