On the following:

(make-directory-internal "c:/")

emacs reports "Permission denied" rather than "File already exists".
This has several consequences.
First,
(make-directory "c:/" t)
should not error, but it does because it does not catch the
"Permission denied" error.
Second, while running
make check
in the current emacs-26 branch, there is a failure
in files-tests.el directly related to the previous point.

Suggestion: modify files--ensure-directory to catch the case
of a root drive and avoid/catch the permission denied error.

Best regards,

Fabrice


In GNU Emacs 26.0.60 (build 3, x86_64-w64-mingw32)
 of 2017-09-19 built on LOBSANG
Repository revision: 4bd6e39c01ebdc815a5d0670922a827295766677
Windowing system distributor 'Microsoft Corp.', version 10.0.16288
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Entering debugger...
<apps> is undefined
previous-line: Beginning of buffer
Configured using:
 'configure --prefix=/MingW64 --with-jpeg --with-xpm --with-png
 --with-tiff --with-rsvg --with-xml2 --with-gnutls --with-imagemagick
 --without-dbus --enable-checking=no 'CFLAGS=-I/mingw64/include -O3 -g0
 -mtune=corei7 -fomit-frame-pointer' CPPFLAGS=-I/mingw64/include
 LDFLAGS=-L/mingw64/lib'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND NOTIFY ACL GNUTLS LIBXML2
ZLIB TOOLKIT_SCROLL_BARS LCMS2

Important settings:
  value of $LANG: fr_FR
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message subr-x puny seq dired
dired-loaddefs format-spec rfc822 mml mml-sec password-cache epa derived
epg epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils help-mode
easymenu cl-print byte-opt gv bytecomp byte-compile cl-loaddefs cl-lib
cconv debug elec-pair time-date mule-util tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel dos-w32 ls-lisp
disp-table term/w32-win w32-win w32-vars term/common-win tool-bar dnd
fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote w32notify w32 lcms2
multi-tty make-network-process emacs)

Memory information:
((conses 16 99907 11069)
 (symbols 56 20316 1)
 (miscs 48 65 98)
 (strings 32 30120 1035)
 (string-bytes 1 759473)
 (vectors 16 14922)
 (vector-slots 8 488682 7533)
 (floats 8 60 110)
 (intervals 56 314 8)
 (buffers 992 12))