There are a couple of things that have to be set so you can read and write any file...

1. The disk (or partition) must be mounted accordingly (ro for read, rw for write)
but you seem to have done that.

2. Every directory (or folder) you open on the path to the file must at least be
executeable, preferably readable as well.

3. The file itself must be readable (and writeable for write access).

I am just wildly guessing here, but presuming that your disk has an NTFS
filesystem you *don't want* to write to your disk - this could destroy the contents.