Drag and drop a file from your Documents folder to your Music folder moves the file. Drag and drop a file from your documents folder to your iDisk public folder copies the file. In the former scenario, both files are on the same Mac volume, so the file moves from the old location to the new one. In the later scenario, the two locations are on different volumes, so the file is copied to the new location, resulting in two copies of the file.
Fortunately you can easily overwrite this default behavior. To copy a file to a different location on the same folder, hold down the option key before you let go of the file. A green circle with a white plus sign will be added to the mouse pointer as a reminder that you are about to copy the file instead of moving the file.
To move a file to a new location on a different volume, hold down the command key before you let go of the file.