Attrib seems to be a Windows shell command to change file attributes.
See :!attrib /?
:!attrib -R % should remove the Read only property in the file. (It works here, the [RO] flag is changed in my status bar).
You will be prompted to reload the file (if you do not have the autoread set): do not do this! and then save with: w!
(Rereading my question, I'm not sure if the Read-Only flag is your problem, because: w! Should work anyway.)
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