[Freesurfer] HELP!A problem of Oracle VM VirtualBox

Dear Guys: Now I'm learning Freesurfer and suffering some troubles.I just installed the Oracle VM VirtualBox and virtualitied a virtual computer with "freesurfer-Virtualbox-linux-x86-stable-pub-v5.1.0-full". When I started the virtual computer and wanted to mount disk partition (eg: F) to this virtual computer.But the system warned that "mont only root can do".What was worse, I could not log in root,because password is required. Even I could not reset the password. Now I'm in trouble.I need your help. Thanks! Best wishes. Yours sincerely BoBo