Reading Folder Contents / Message Saying Cyclic Redundancy Check

I'm having some trouble with my hard drive. It all started when a friend wanted me to format his external hard drive, so I copied all of his data on to one of my drives and formatted his disk using a utility that came with it (he wanted it as FAT32). Anyway, I did it kind of in a hurry and it seems that I also copied the "System Volume Information" and "Recycled" folder on his disk into a folder on my disk by accident. So now every time I want to enter the folder where I put this stuff, windows says that "the disk is not formatted" and asks me if I want to format it now. Also, and more importantly, the stuff in some of my other folders has "disappeared" and is not showing up in the windows explorer. The drive is being very slow when I try to access some folders but not with others and when the computer is idle it makes a continuous "click......click.......click..........click" sound, quite slowly. When I try to copy files from certain folders, I also get an error message saying "cyclic redundancy check" (I'm not sure if that is the exact translation - my version of winows is in French). Now I'm not sure exactly what the problem is here because prior to copying this data, my disk was fine.