ADD does not allow you to change the Destination, it is by definition the same as the Source. (It seems you are saying that is not the case, if that's what you are saying then you need to show us a screen capture)
But each time I load photos from a disc, I make a folder for them on Drive F and copy them from the disc to the new folder. I then want to add these photos to the catalog which is presently active. I want to add them without moving them
I really don't understand this. Copy to new folder, ok, then ADD without moving them, ok, so that's really what ADD does. So what is the problem?
When I use the Import - Add command, LR allows me to choose the source, but immediately chooses the destination as "My Catalog" Drive C. Although there is a little checkbox next to Drive C, it is inoperable and I can't find any way to change the destination. If I use other commands such as Copy, I am able to change the destination. But each time I load photos from a disc, I make a folder for them on Drive F and copy them from the disc to the new folder. I then want to add these photos to the catalog which is presently active. I want to add them without moving them..
Copy: Copying the pictures to a destination directory and adding to the Lightroom catalog.
Add: Only adding the files to the Lightroom catalog. As there is no change of destination involved, there is no need to enter a destination.
For the rest, your configuration is rather complicated and non standard from a Windows point of view.
It doesn't matter which one of your hard drives the catalog is located on. If that catalog is the one that is active then it is the one that will contain references to the imported images. When you use the Add option in the import dialog the images are added to the catalog from their present location. You cannot specify a different destination when using that option. When you use the Copy option, then you are able to specify a destination in the right-hand column of the import dialog. You need to choose the appropriate drive and master folder in the folder tree that is displayed. Then specify the name of the subfolder in the area highlighted:
You mentioned changing the destination of the catalog. Either that was a bad choice of words or you don't understand how the catalog works. The catalog is located in a single location. Normally it doesn't move anywhere unless you choose to do so manually. You can control the destination of your imported images, and Lightroom will import them to the destination on any hard drive. But they are managed using the catalog which remains in a single location.