Do you have more than one drive on your computer? Are you manually saving documents to the D: drive? Either way you can just move your "My Douments" folder to a different drive. Just right-click on "My Documents" and click "Properties". Just enter the new path in the "target" field or click "Move" and navigate to the destination folder. If there isn't one you can create on the drive you wish by navigating to it and clicking "New Folder" and giving it a name. Click OK. If you used the "Move" button then click "OK" again.
You will be asked if you wish to move your existing documents to the new location. You would normally want to answer "YES" to this.
Once the files are done moving your "My Documents" folder will reside at the new location!