Cannot copy files from internal HD to external HD

I booted my laptop using Knoppix 6 and it can see the contents of my internal HD. I can even open a file play a WMV file or open a document. BUT, when I tried to transfer to the external drive connected via USB, I am getting a Read-Only Filessystem error message which seems to me is pointing to the external drive. How do I fix this??

thanks in advance