I think your husband is right
Drag and drop...cut and paste...whatever method you choose...will work for moving files between partitions.
Do you have two separate drives...or two partitions on the same drive? Only reason I ask is that two partitions on one drive is slower in moving, IME.
IMO, the C: drive should only contain a minimum of data files, if any. Windows and programs installed should be on the C: drive just for the sake of expediency, although some choose to install programs on a different partition.
Just make sure that you can see this D: partition in both My Computer and Disk Management and that it's either FAT32 or NTFS file system.
You should probably also run Disk Cleanup to get rid of unwanted trash on the C: drive.