Need help moving files to external HDD

Hi,

I am new to Mac. I just need to move some files to my external HDD. When I open the folder these files are in they all highlight and I am able to drag them to my external hard drive. However, once I get them into the hard drive folder a red circle with an "x" comes up indicating that I cannot drop here. I also noticed that the external hard disk folder has an icon that is a pen with a circle and a line thought it. I am thinking that this means that I cannot write to that drive. I am able to move files from the external to the Mac.

Can anyone please help!??

Thanks!