The problem is solved by going into Windows and turning off this feature, with little noticeable impact on boot times. Could it be it was introduced to make it harder for Linux newbies to dual boot and access their data?
Here is one version of the instructions to do this that can be found by searching:
Note that the increased booting time described is considerably exaggerated. It is also not required to use Ubuntu as your Linux distribution.There is a new feature in Windows 8 called Fast Startup. If this feature is enabled (which it is by default), Windows 8 does not actually completely shutdown when you choose shutdown. Instead, it does a "hybrid shutdown". This is something like hibernating; it makes booting Windows 8 back up faster. So, you need to disable this feature to be able to shut it down properly, and be able to mount the Windows partitions. To do this, boot into your Windows 8 and:
Note: disabling Fast Startup will most likely make your Windows 8 take a longer time to boot. There are no "exact" numbers, but let's say that if it took you 10 seconds to boot into Windows 8, it will now take you 50 seconds after disabling this feature.
1. Open Control Panel in the small icons view and click on Power Options.
2. Click on Choose what the power buttons do.
3. Click on Change settings that are currently unavailable.
4. Uncheck Turn on fast startup (recommended).
Click on the numbers above to see screenshots.
Click on Save changes. Now, shutdown Windows 8 and boot back into Ubuntu.
If you still aren't able to mount without getting errors, you may need to turn off hibernation completely. Open an elevated Command Prompt (right click on the shortcut, click on “Run as Administrator”), and input:
powercfg /h off
Source: Fast Startup - Turn On or Off in Windows 8.