You can't move space from one to the other, but you can store things on the D drive. Think of the drives like big storage containers, one is a shoebox and the other the box from a case of paper - there's no way you can "add" the space from the case of paper to the shoebox, but if there are too many things in the shoebox, you can put them in the case.
(Of course, as Daisy mentioned, you do also need to back up somewhere else if that's what you've been using your second hard drive for.)
I used to have a tiny C drive so I would load new programs onto the D drive and had no problem, but I guess moving programs already in place might be more of a challenge in missing pieces when you move them (I just know enough not to try it
). But there's no reason you can't have files on the other drive. If you store all of your scrapbook pages in a particular folder on the C drive, you can just move that folder to the D drive. Same with your elements, etc.
(I also just asked my husband who spends 90% of his work time repairing laptops, and he said there's a good chance that the D drive is actually a partition of the original drive [think of that like the organizer for holding silverware we put in our drawers - you COULD just throw all of the silverware in one drawer, but it's a lot quicker to find the forks if you use the organizer]. That doesn't really change my answer, but just so you know, there may not actually be two separate hard drives in the laptop.)