Disclaimer - For me the Apple Support Communities are an international user to user technical support forum. As a man from Mexico my first language is Spanish. I do not speak English, however I do write in English with the aid of the Mac OS X spelling and grammar checks. I also live in a culture very different from that found in most 1st world nations, such as that in the two dominant North American countries. Written language has no tone. When offering advice in the ASC, my comments are not meant to be anything more than helpful and certainly not to be taken as insults.
What are you trying to copy & paste?
To copy from the internal drive to an external drive you can open two finder windows, one for the internal drive and one for the external drive, then drag & drop the content that you wish copied. This will copy the content and not have effect on the original.