Haddo, here is an image of a complicated Disk Management environment that is useful for illustrative purposes:
This screen shows us that there are 2 physical hard drives in the system, Disk 0 and Disk 1. Disk 0 has 4 partitions, C, E, F, and G. Disk 1 has 3 partitions, H, I, and J. Note that the C: partition is marked as Healthy (System). This is not only a bootable partition, but it is *the* partition that the operating system is currently booted and running from. Look at H:, which is marked as Healthy (Active). This is a bootable partition, meaning that it contains an operating system, but the OS on that partition is NOT running. That other active partition will also not show up in Explorer because Windows only allows 1 bootable partition to be shown in Explorer and that is the one that it is booted from.
So, your D partition is Active, but not booted and that's why it doesn't show up in Explorer. In order to see that data while booted from the C partition, you first need to mark the D partition as inactive. I don't believe that Disk Management will let you do this operation. You will need to use the diskpart.exe command line to perform this operation. If you need help with that, I will need to know all about your drives and partitions in order to insure the correct partition is marked inactive.