The essence of the problem has to do with ownership of files and folders.
Windows can have multiple users on a machine and the expectation is that they can each have files which other users cannot view or use.
This user system leads to admin users and other classes who can use takeown.exe and cacls.exe to change permissions on the file system and give the ability to access certain system and user folders.
If you have no expectation of security on your computer then you can give permission to each file and folder to "Everyone" which is a user privilege that is very low in the security hierarchy. Windows isn't designed to allow this - some folders will revert to system access repeatedly - and this will allow malware to gain access easily too.
Another way around this is to create a folder which you use to save every personal file into, in a category of sub folders, and then backup this 'master' folder which contains all your files.
You might also like to investigate XXclone which can backup the entire hard drive to a USB device (or internal drive) in file mode - and each backup only copies the changed/new files and gives you a mirror and bootable backup. The benefit is that after the initial backup then each subsequent backup is very swift, of the entire system. You can access every file in the backup the same way you can on your master drive. The backup hard drive only needs to be as large as the files, so you can backup to a smaller HDD than the primary c: drive.
Robocopy is a command line tool that is similar to xcopy but it also skips existing files in the backup, so is swift. It has mirror backup capability too.
Similar to xcopy, it cannot copy files and folders that your user does not have permissions to access.
I hope that illuminates the issue somewhat...