Você configurou pastas compartilhadas e instalou as ferramentas do vbox no convidado? Parece que você está tentando acessar a unidade C: por meio do compartilhamento de rede CIFS "normal". É muito mais rápido e mais difícil de quebrar, se você usar pastas compartilhadas:
Boot the Guest and open a terminal. Create a folder where you will mount it on. E.g. in your home folder. I will use
~/host
for the mount point. Now mount it with the following command:sudo mount -t vboxsf share ~/host
Note that with this, the default mount options are used and all files are owned by root.
This can be changed by adding some mount options. Options are passed on with the -o parameter. You can use multiple options with one parameter, seperate the values with a comma. See the man page of mount for more info on which options you can use. The User Manual also notes the options compatible with the Shared Folders. To mount the SF so that you are the owner of the files, use this command:
sudo mount -t vboxsf -o uid=1000,gid=1000 share ~/host