Sounds suspiciously like it is formatted at NTFS (Windows) but given that you successfully copied files to it, it shouldn't be the problem.
With the EHD plugged into the Mac and with it mounted on the desktop, reboot the Mac.
Try to copy stuff to it.
Any error messages?
I've recently downloaded some mp4 files and have successfully copied them to my EHD so that I can watch them on TV. My Mac had to be re-set (unplug the power cord) because my keyboard and mouse weren't working and ever since then I haven't been able to copy my mp4 files to my EHD. Would it have something to do with the computer being re-set? When I go to copy the file to my EHD I get the "circle with the line through it" and it doesn't recognise or open up my EHD so I can copy to it. Any ideas anybody?