On a 7800DX with firmware 2.32c:
The FTP public directory list the USB partitions mount names as a subdirectories, files transfered to these subdirectories appear in a directory called 'share' on the associated USB device partition - great.
You can however, transfer files to the root of the public directory, which appear to be stored on the router itself. If this is right, then what would be the impact of an FTP user filling this space up? Could it impact on the running of the router and maybe crash it?
ps. Also I think the availability of a public FTP folder should be an option.