If these are important to you then make sure you have at least TWO copies of the files.
You have bought one external hard drive, but I would now go out and buy ANOTHER.
First COPY the files from your PC to the EHD (using any method you like). These are your BACKUPS.
Then MOVE the files from your PC to the second EHD, and these are the files you are going to use.
External Hard drives contain moving parts and they can easily crash or be dropped or be lost or stolen so it important to have the files backed up on a second external hard drive.