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.
("\\.[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/address@hidden", where "dirve_c" was put in.
I attached the log file, and it may be anchored by find "c:/plink" and "drive_c".