Altering "sharing and permissions" on the Imac.

I have a portable hard drive that i want to transfer files to off the mac's HD. Having looked at the help section on the Mac, i realise that the file you want to send has to be "read & write" or "read only" (no problem there), and the location you are transferring the files files to has to be "read & write" or "write only". That is the problem; the portable HD comes up as "You can only read". How do I change that permission to "read & write" or "write only"? I am logged on as admin, but when I follow the advice in the help pages, I get this;


However, in the "get info" window for the portable HD all I can see is this;

There is no sign of my username or the pop up menu for the permission settings. I've tried going through the "system preferences"-"accounts" path as well, but no joy there either re permission settings. Any clues out there? I'm fairly new to the Mac, so an idiot-proof explanation would be appreciated !