My apologies, I may not have made myself completely clear.
From an earlier post I mentioned that my C drive is slow and nearly full, and that I am therefore wanting to save FTM files to a separate custom directory, in my case D:\Dropbox.
FTM files are being saved without problem to D:\Dropbox after using Tools>Options but media files are being saved to C:\My Documents\Family Tree Maker\(My Tree)\Media.
From what you are saying, should I copy my media folder to the relevant D:\Dropbox subfolder? Would FTM then progressively update the relevant D:\Dropbox media subfolder? If so, Dropbox would then sync the exact FTM data I require, and all FTM files would be stored on D drive.