Why disable Fast Boot on Windows 8 when having dual booting?
Fast boot explained with an image:
During fast boot the system loads the hiberfile and does not use the files on the filesystem.
So the biggest problem, and the worst problem you can have on a computer, is data loss: if you create a shared NTFS data partition the hibernation may maintain the file structure so if you try to save a file from Linux into the NTFS partition it will get lost on Windows reboot as it only remembers the old file structure. See this Ubuntu Forums thread for more information.
That alone is reason enough to never touch fast boot in a dual boot setup.
Two relevant discussions from Super User:
Hibernating and booting into another OS: will my filesystems be corrupted?
What do I have to take care of when hibernating both Ubuntu and Windows (dual-boot environment) Make sure to take note of this answer by Gilles.
In general: if a disc is mounted when using fast boot, Windows puts this disc and its content in a hiberfile. Any changes made to the system are gone when the hiberfile is restored. This includes mounted external discs. Ubuntu will refuse to mount a disc that has a hiberfile.
One of the bad messages you see in these topics is getting a "disc has errors" when booting Windows.
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 load Windows.
There are however 2 ways to disable this so you can share files between both systems as described in Installing Ubuntu Alongside a Pre-Installed Windows with UEFI