(1) The 'fritz.box' is not a public internet address. It is a rather clever way of this router (i.e. the FRITZ!Box) to use it's own DNS server to point to itself so that the users don't have to remember the IP address (e.g. 192.168.178.1).
(2) The
http://ftp://... is a (known) weakness of the forum software: it turns 'ftp://...' into '
http://ftp://...' TIP: Encapsulating the address in single quotes should work (e.g. 'ftp//fritz.box/SanDisk-U3CruzerMicro-01/exchange/').
(3) The 'ftpput' and 'ftpget' applets for BusyBox do not support the listing of file names, so it's not as comfortable as the use of a browser (or the "standard" FTP client). But one can still use them provided one knows the file name, e.g.
ftpput fritz.box SanDisk-U3CruzerMicro-01/exchange/FileToTransfer FileToTransfer should work to send a file to the USB pendrive on the FRITZ!Box and
ftpget fritz.box FileToTransfer SanDisk-U3CruzerMicro-01/exchange/FileToTransfer should work to retrieve a file. Please note the rather weird order of the local and remote file names in 'ftpput HOST REMOTE LOCAL' and 'ftpget HOST LOCAL REMOTE'.
(4) A rather minimal FTP client is available via the 'cftp-0.12.tcz' extension. I had to use
echo 'set passive 1' > ~/.cftprc to make it work in my setup, but then
cftp fritz.box was working quite well. Mind you the help screens are all a bit minimalist, but it allows to tag a group of files (with 't') and then download the lot (with 'D'). HINT: to see the key mappings use 'H' followed by 'K'.
EDIT: Added (4)