You should really be thankful that the operating system are clever enough to
protect you with this unsafe setup, since otherwise you risk destroying your disk.
Fast boot and dual boot cannot both coexist.
I quote from an Ask Ubuntu post
which is for Windows 8 but applies also to Windows 10:
Fast boot in Windows 8 is a way to boot the system faster because the needed data to boot (Drivers, User session, etc..) are stored in a
hibernation file (
hiberfile) and are loaded when the boot process
begins, saving the user between 40% and more boot time.
Since Hibernation mode is a way to "freeze" whatever you were doing
before shutdown and loading it again when you start the computer (this
includes opened apps, sessions, drivers, the last office doc you were
editing...) it creates a problem when you want to copy stuff from
Ubuntu to Windows after a hibernation, or "fast boot" as it is called
on Windows 8 because anything that changes between hibernating and
booting again is lost.
Basically, if you shutdown Windows 8 (hibernate mode) and then go to
Ubuntu and try to copy something over to Windows 8 like an mp3 file
(assuming Ubuntu does not throw any warnings about it being in
hibernation mode), when you boot Windows 8 again, the mp3 file will
not be there because it was not frozen during the shutdown procedure
you did on Windows 8 before going to Ubuntu. Since the mp3 file was
not there BEFORE shutting down Windows 8, it was not saved in the
hiberfile and thus will not be restored after you do a Fast boot to