Open a command prompt and type:
chkdsk /f /r <Enter>
It will say "A disk check could not be performed now, would you like to schedule....?" Press Y for Yes.
Exit the command prompt and restart your computer. It will say "A disk check has been scheduled, press any key to abort" or something like that. Do NOT press any key, let it do a disk check.
Then try your backup again.
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