I installed a external USB hard drive with win 7 32 Professional, but carelessly did not format it prior to moving approx. 500 GB of media. I never got a msg to format until the drive was unplugged from windows explorer and I wanted to install it on another PC to transfer other media files. Now I can't transfer any files off (unrecognized file system). I finally accepted I was going to just have to format and re-transfer the files, but it won't even let me format the drive! How do I fix this?