I have two HDD. One has the Win 10 and the other has the Kubuntu 18.4 version. I flashed the Kubuntu OS before two months. I had the same problem. Which is that I cant able to copy or paste in the NTFS Win 10 bootable HDD from Kubuntu. So I flashed the same Kubuntu OS in same HDD where I installed the Kubuntu before. After that I tried to copy and paste in NTFS HDD and it worked. Later I messed up with Kubuntu and again I installed same Kubuntu OS which I had before. Now facing the same issue. One thing I noticed that, when I faced the same issue I saw the KDE wallet asks to save the Wifi password while connecting and google drive didn't get connect in Dolphin. And also the KDE wallet didn't head up when it worked well. What would have caused this problem.
With Ubuntu, you should by default be able to read and write on a partition that is formatted in ntfs, the file system of Microsoft Windows. The condition, however, is that the volume should be "clean", i.e. in a properly closed, cohesive state.
In a dual boot system, the ntfs partition may not be clean if Windows "Faststart" is enabled, or if the Windows system is hibernated instead of powered down. In these conditions, files on the volume may still be open. While Windows will cope with that on the next restart. Linux will mount such drives as read-only, or not at all.
Thus, to solve the issue:
Next time you boot into Ubuntu, the drive should mont without issues.