Welcome to the Seven Forums! dessire You're in kind of a predictament no less.
When you are not able to move files from one drive to another then you have to start looking at alternatives like removable media if you are not able to redownload the same files later following a wipe of a drive.
One fast option would be shrinking the D volume down enough to create a second small partition at the back end of the drive for safe keeping what you are not able to burn to disk, copy to flash drives, or download again. Once you have them backed up you will then be able to reformat D and move them back there.
If goes well you will be able to delete the temp partition and expand D back to where it had been. Note that other options for backing everything up would be advised first however since partition changes can see mishaps at times too.
If you find Windows needs a fresh install at some point and wipe C you can that to restore files and settings for Windows and the programs you need to reinstall again.
W7 Ultimate x64/W10 Pro x64/W11 Pro Triple Boot - Main PC W7 Remote PC Micro ATX W7 Pro x64/W11 Pro
CPU
AMD Phenom II X4 975 Deneb 3.6ghz - 965 2nd remote pc
Motherboard
Gigabyte GA-790XTA-UD4-Gigabyte GA-880GM-D2H remote pc
Memory
Kingston Hyper X DDR3 1600 1.5v 16gb - Hyper X Fury 8gb 2nd
Graphics Card(s)
MSI HD Radeon 5750 1gb - MSI HD Radeon 6450 on mini tower
Sound Card
Creative Labs X-Fi Xtreme Audio P - Realtek onooard 2nd case
Monitor(s) Displays
ASUS VW199T-P 19" HP 2082a Main-HP 2082a 20" remote pc
Screen Resolution
Asus 1440x900 - HP 1600x900
Hard Drives
WD Black 1TB HD per OS W7, W10, and pending W11 presently on 500gb OS Drive - Pending Triple 1TB HDs for Spanned Storage/backup volume
Single 2TB external USB enclosure, single 1TB System 7 Host/Boot drive, Pending 8TB external HD for system image b
PSU
Corsair 750TX - primary / Corsair CX600 - second
Case
Antec 900-2 - SSD compatible / NZXT Vulcan mini tower