It should be "c:/Users/username/HOME/.
emacs.d/autosave/Rfiles/!plink!!username@host!!!opt!bee!home!username!test!coxph_bench.r.~2~",
because it is in server in not in "c:/plink...." and there should be "!drive_c"
added.

On Sun, Nov 26, 2017 at 3:18 AM, Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Shuguang Sun <shuguang@gmail.com>
> > Date: Sun, 26 Nov 2017 00:08:42 +0800
> >
> > Windows 7
> > GNU Emacs 27.0.50 (build 1, x86_64-w64-mingw32) of 2017-11-24
> >
> > I didn't set tramp-backup-directory-alist which means the fiile will be
> backuped locally instead of being backup
> > in server side.
> >
> > backup-directory-alist:
> >      ("\\.[rR]\\(md\\)?$" . "~/.emacs.d/autosave/Rfiles")
> >
> > When I open a file in tramp and edit it, the backup file will be and was
> backuped in the local direcory:
> > ~/.emacs.d/autosave/Rfiles (c:/Users/username/HOME/.emacs.d/autosave/Rfiles).
> However the backfile is
> > named
> > "c:/Users/username/HOME/.emacs.d/autosave/Rfiles/!
> drive_c!plink!!username@host!!!opt!bee!home!username!test!
> coxph_bench.r.~2~",
> > where "dirve_c" was put in.
>
> Maybe I'm confused, but what did you expect to happen instead?  When
> files are backed-up in a single directory, the drive letters cannot be
> kept because Windows doesn't allow colons in file names.
>