Re: What is wrong with my external HD?
'o.O First off, don't panic yet, and don't try to format the drive yet. The reason you couldn't format the drive is because the files were being in use by Windows, so it couldn't complete the process. Try simply restarting the computer with the HDD plugged in, and if you still have the same problem, run CHKDSK, a Windows utility that fixes problems similar or identical to this.
If I remember this right...
1. Press the Windows key and R, then type in "Command".
2. Type in "CHKDSK X:" X being the letter of your external HDD.
3. Let it do its thing; it will likely tell you to restart your computer for it to complete the process, which is OK.
If you still have problems, scan the HDD with Scandisk, and if that picks up nothing, then I am out of ideas based on the info I have, with the exception of a wrong setting on iTunes or something.