Keep it simple .
Set the NEW Backup USB HD letter to U: , dont use the default drive letter as that can change , it the USB drive letter changes, backup might fail
Check the backups (dont assume they work) . Do test recovery of recent & old files

Either use a file sync program, robocopy , or Windows Backup (disk image option)
Because you have stuff all over, & dont know where some data is, you need to backup the whole HD . Otherwise just a backup of the 'user' dir would do the trick(usually).