First, there are two "fast-boot" features:
- In the firmware -- The firmware's fast-boot feature is relatively harmless, although it's not necessarily completely harmless. Details vary from one firmware to another, but typically, the firmware's fast-boot feature bypasses certain types of hardware initialization. It might do only minimal or even no USB initialization, for instance, which may make the computer ignore USB disks as possible boot devices, which is obviously a problem if you're trying to install using a USB flash drive.
- In Windows -- The Windows feature is usually called "fast startup," but for dual-booters, it might be better called the "damage shared filesystems" feature. What it does is to turn the Windows shutdown option into a suspend-to-disk operation. Of most importance, filesystems are not flushed or fully unmounted, which means that when they're accessed in Linux (or any other OS), it will look to them as if Windows had crashed. When Linux repairs damage (if it can) and Windows is re-booted, Windows will get confused. When booting either OS, filesystem damage can result. You must disable this feature in order to safely dual-boot Windows 8 with Linux. See this Windows forum post for details on how to disable this feature.
Second, the "unknown filesystem" message in GRUB is probably not caused by either of these features, although it's conceivable that if filesystem damage were bad enough, it would cause GRUB to complain. Thus, you might first try disabling the Windows fast startup feature and repairing all your filesystems (including the EFI System Partition, or ESP).
If that doesn't help, then the problem is most likely one of a mis-matched filesystem -- that is, GRUB is configured to read its configuration file from one filesystem but it's finding another one. Re-installing GRUB might help with this. Boot Repair is the easiest way to do this, but you say you've already tried that and it's not helped. Thus, you might want to try another boot loader. The easiest to try is rEFInd. You can try it fairly easily by downloading the USB flash drive or CD-R image and preparing a suitable medium. This might boot Linux and Windows as-is, depending on your configuration. If it works, you can install the Debian package version in Linux.