In my experience adempewolff's popular and helpful answer above was necessary, but not sufficient, to allow me to mount my Windows NTFS partition for writing with Ubuntu. As instructed elsewhere I turned Fast Startup off before trying to install Ubuntu and I removed the Hibernate Option on the "Shutdown" menu, too.

I still couldn't write to my Windows partition from Ubuntu.

I found I also had to boot Windows 8.1, start a Windows authorized command line (right click on the Windows button on the bottom left to get to this option easily), allow it throught the Windows authorization box, and then enter the command:

powercfg /h off

You can check the results with:

powercfg /a

After making this change I was able to freely access the Windows 8.1 partition from Ubuntu, whether I quit Windows by shutting down or by restarting.

I found that I was able to later reverse this and still access the partition (but keeping Fast Startup unchecked at all times, as above, and never asking for Windows hibernation of course). The command to reverse it is, predictably:

powercfg /h on

I assume something was left over from Windows installation that needed to be cleared by booting with hibernation turned off in this particular way. There should be no reason to undo the first step like this as far as I know--it may provide a bit of extra safety to leave powercfg /h off.

See the Ubuntu man page for the Windows NTFS handler for a bit more information.

In my experience adempewolff's popular and helpful answer above was necessary, but not sufficient, to allow me to mount my Windows NTFS partition for writing with Ubuntu. As instructed elsewhere I turned Fast Startup off before trying to install Ubuntu and I removed the Hibernate Option on the "Shutdown" menu, too.

I still couldn't write to my Windows partition from Ubuntu.

I found I also had to boot Windows 8.1, start a Windows authorized command line (right click on the Windows button on the bottom left to get to this option easily), allow it through the Windows authorization box, and then enter the command:

powercfg /h off

You can check the results with:

powercfg /a

After making this change I was able to freely access the Windows 8.1 partition from Ubuntu, whether I quit Windows by shutting down or by restarting.

I found that I was able to later reverse this and still access the partition (but keeping Fast Startup unchecked at all times, as above, and never asking for Windows hibernation of course). The command to reverse it is, predictably:

powercfg /h on

I assume something was left over from Windows installation that needed to be cleared by booting with hibernation turned off in this particular way. 

There should be no reason to undo the first step like this as far as I know--it may provide a bit of extra safety to leave powercfg /h off.

See the Ubuntu man page for the Windows NTFS handler for a bit more information.