I was trying to run Partition Magic and I got the following error.
I selected "Yes" and a complete partition with 370GB capacity disappeared from the File Explorer.
So, I ran Disk Management to check if the missed Partition is present there. It is there.
I tried to give a letter (by using Disk Management & also AOMEI Partition Assistant) to the partition to make it re-appear in the File Explorer, in which I failed.
So, I merged this drive with a 30GB unallocated drive which I had shrunk to install Ubuntu as Dual Boot. That solved the problem. I was able to see the drive back in the File Explorer with 400GB space.
Does this error has something to do with the Ubuntu Installation Problem which is one of my other posts, which I am dealing at http://answers.microsoft.com/en-us/windows/forum/windows8_1-windows_install/problem-with-dual-boot/651c7b44-014b-4088-a4bb-49be78aa08e3 ?