DO NOT do this as described in the first sentence.

If you just copy and paste your files to the D: drive Lightroom will lose track of them and you will have to start your catalog over. That will lose all your edits.

Yes, you want to move your photos from C: to D:. However, (1) you can do that within Lightroom and if you do it that way Lightroom will know where your files are; (2) you should back everything up before doing anything.

I agree that C: should contain only system files and program files. Not data.

I agree that you should copy all your Lightroom catalogs onto the disk that holds your photo files. It does not need to be on C: It can be anywhere.

Back up everything first. Move using Lightroom second. Move the LR catalog to D: third. Test Lightroom to see that everything got transferred successfully. (If you rename the root folder of your photo folder structure, that will hide the photos from your old Lightroom catalog. You can then use Library => Find All Missing Photos to see if there is anything you didn't get yet. If there is a problem you can find things in your old folder structure). Once you have everything backed up and working properly you can delete the root folder in your C: photo folder structure.