How to install guest addition in sage:VERR_PDM_MEDIA_LOCKED

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I am very new to sage. I installed sage in my windows operating system using Oracle VirtualBox. I want to access text files in windows through sage. When I try to set shared folder, I saw that I should install GuestAddition. I tried several time but every time it give me VERR_PDM_MEDIA_LOCKED error. I am not sure what to do. I read oracle virual box addition help and that didnt work. I search through web but couldn't find any clear answer.

Since text files contain data, I really want to set shared folder. So help me.....