The way Windows enumerates volumes, D: would be a partition on the first
drive. It is probably your recovery partition. Setup enumerates partitions
on the first hard drive, then the optical drives, then the next internal
hard drive, then such things as card readers. An external drive is assigned
the next available letter. I doubt that you will ever see a computer on
which the D: partition is an external drive. It is possible, but has to be
intentionally reorganized that way by manipulating drive letters in Drive
Management. You would remember having done it.
As an example of how Setup enumerates the storage devices, on my system,
with two hard drives and two optical drives, I have one partition on the
first drive and it is C: Since there are no more partitions on the first
drive, the two optical drives are D: and E:. The second hard drive has a
single partition and is F:. It could not have become D: during Setup
because optical drives are enumerated after all the partitions on the first
drive and before any partitions on the second drive. I then have four
volumes that are the card readers, G:, H:, I:, and J:. When I plugged in my
external usb drive the first time it was assigned K:.
That is why I am confident that your D: is a partition located on the first
drive with C: and not a second internal hard drive or external drive.