When you use some backup product to back up your data (for example, a folder), the archive attribute will be cleared (meaning that this file has been backed up). If you make some changes to the data after you back up this data, the archive attribute will be selected automatically to indicate that the data has been changed since the last save. So, next time the backup utility runs, it will check all the files archive attributes to see which data has been changed since the last backup.
You can see this attribute by clicking Advanced button on the General tab of the folder's property. It is the "Folder is ready for archiving".
So, if you have more than one backup product to regularly backup your files and after the initial backup some files have been changed. These changed file's archive attribute will be selected. One backup utility runs and finds that the files have been changed, it will back up these files and then clears the archive attribute. After that, another backup utility runs, however, it will find that the archive attribute is not selected, so it will see this file as unchanged since the last backup. Conclusively, after a long time, you will find the backup files are not the same within the different backup products.
The total difference between these two options is, copy backup copies all the files that you select, but does not mark each file as having been backed up (in other words, the archive attribute is not cleared).
So since you're having an external hard drive, and want to back up my computer to the drive, I would recommend your run a Windows Backup so that all your files are backed up and you will preserve all the applications files including log files on the computer, rather than copying selected files.