Trying to scan all files on hard drive and keep getting 400 files that will not open

I am running Linux Ubunto 16.04 LTS and I have the sophos free anti-virus anti malware version and from terminal I write the following command savscan / -all and it can't open 400 files, so how can I scan all files on hard drive and how can I scan all files on a thumb drive when I am working off a hard drive on the pc. I basically want to scan everything on the pc before I use it without any files that can't be opened.