Were they put on that drive from within Linux or within Windows?
They're likely there, but Linux doesn't necessarily understand that the drive is related to your user login from Windows and it probably just believes it is data in general.
The drive would not be called D: in Linux it will likely be called something else depending how it was mounted, and if it was mounted.
From a terminal or command prompt if you issue the "mount" command it will show you all mounted disks.
Likely you'll see stuff like:
/dev/sda1 on / type ext4 (rw,errors=remount-ro)
And then likely other lines. Note that the letters /dev/sda
may be different, and that's up to your hardware situation; and further the number /dev/sda1
may be different, also up to your hardware as well as how your disks are partitioned.
Likely you'll see multiple /dev/sd lines some with one letter, others with a different letter.
The difference in the letters is usually different physical disks; and therefore the differences between what you'd call C: and D: under Windows.
If you notice that "/" is mounted on one letter, then Linux calls this your root file system and so that is likely C: The other /dev/sd<letter> is then likely D:
This report however should tell you where that other disk was mounted, as an example /media/<some very long number> where you can go to the /media/<very long number> directory and then see files there.
The other situation may be that the disk is there, but not mounted and thus not visible to Linux normally and you'd have to mount it to see the files. If this situation exists, then there's one or more commands you can use to get a report on the physical disks in the system, one of them would be:
sudo lshw -class disk