i'm so confused because my Os drive letter is c but the boot device is letter D
i think the windows use the boot device is my os letter whenever i used my startup repair my OS will detect as letter D but its not
how can i get rid of this problem
It's not a random option.
Your OEM installed W7 with the boot files in their own boot partition, not in the C partition.
EasyBCD is just telling you where they are. Why do you consider that's a problem ?
Also, BCD backups aren't made unless you make them explicitly... and the default to being saved in your My Documents folder. What's more, if you were to select a different folder, the next time you launch EasyBCD that previously used folder will be the new BCD backup save location.