I installed GIMP 2.10.0 yesterday. When I try to open files, there are a few folders listed, such as "Documents" and "Pictures" as well as the letters of the hard drives. When clicking on a listed folder, the contents show up fine, but when clicking on any of the hard drives, it shows absolutely nothing. I tried running as administrator, but to no avail. Restarting the PC did nothing as well.
This could be a bug in GTK+, unless it is something deliberate (similar to the default Windows Explorer setting which hides system directories from the user).
For the moment it's being tracked in the GIMP Bug Tracker: https://bugzilla.gnome.org/show_bug.cgi?id=795855
Typing a drive letter - i.e. c:\, d:\, ... - in the location bar is a workaround, as this open a dropdown of possible completions.