Seeing you mentioned that you used MBR more info on your hardware could also help others to help you better:
At first sight one would think what does hardware have to do with an error about failed un-mounting, but you never know.
Providing more info as what is needed can only help others to help you more 
PS:
I just noticed from your Screenshots, that those were taken from within M$ 
Are you using a VM to boot your Manjaro?