I had a problem with an external HD where I had accidently dropped it and in the end I could not fix it. Have you, by any chance, dropped it? If not, I would try running a defrag using either the windows 7 one or, preferably, Auslogics free one. If there are only a few bad sectors maybe you can ignore them.
How to get Windows 10 now: how to download and install Windows 10 even if GWX.exe is missing