... now I need help! If possible!

I have a Max_Drive 300 gb external drive with usb connectivity. I run on a macpro and usually have no problem with other usb devices going between pc/mac, such as a mp3 player or thumbdrive.

For some reason I can mount the hdd but I can't copy to it...it is acting as it is read only but when I open it in the disk utillity I states it is read/write.