If your "PC" is running Windows 7 and newer - "there is an app for that", Windows Backup and Restore. You can also use your external disk as ordinary file location where you copy the items you want.
I pretty much want to copy my HDD and put it on external one.
Do you want just to have image backup copy of your hard disk (so you can restore it in case something goes wrong), or you want to run Windows off that external drive?