I think this might have something to do with the fact that Windows 10 is still new and there has yet to be an update in Ubuntu to accommodate the following issue. Nevertheless, I think everyone should be aware of this so we can hopefully find a solution for it. Here we go!
I had Windows 7 and Ubuntu 14.04 dual boot just fine and was able to access my Windows partition (for music, pictures, etc) without any problems. However, upon doing the free upgrade to Windows 10, I had to re-install Linux on the remaining partition (no big deal). But now, the following happens. I can only access the Windows 10 partition if I first boot up Windows 10 and then "Restart" into Linux. Even if I shut down Ubuntu and go back into it, I'll still have access to the partition because the last function I performed in Windows 10 was "Restart." If I go back into Windows 10 and click "Shut Down" and then power back up into Linux, that's when I get the mounting error stating "Unable to access 'Windows 10'"
If you guys have any insight on how to fix this issue or if you believe that Ubuntu has to release an update to correct the problem, please respond either way! Thanks! :)