Transferring files to MyBook and deleting transferred files on my computer

I need to delete some files from my computer after I have backed them up on my MyBook. Right now the files are being deleted on both devices. Any help is greatly appreciated.
Thanks

You will need to either use a different backup application or to manually drag and drop your files into a regular folder within your WD My Book.

Do you suggest that I delete what I have copied to the HD?
What other back up application would perform better than Time Machine?
Why can’t I modify something that has been copied over?
It tells me that I need an administrator name and password. How do I create or bypass that?
Sorry about all the questions but I really need advice. I thought it would just be simple.
Thanks for your help