Can you verify whether the read-only state is due to the file permissions, the share permissions, or the account being used to access the share?
While I’ve never run into the “please verify the SHA256 hash” message before another user did here:
However, it’s unlikely a read-only scenario would cause this message. It might be worth “manually” checking one or two file sizes on the destination to see what they are shown as. This could possibly narrow down whether there’s an actual file size issue (maybe filesystem issue as you said) or just a REPORTED one (maybe your destination share / FTP / SFTPs / etc. server has a problem reporting the real file size).