Solution:
I totally missed the new "Files clipboard" extension on the new Marketplace at first.
With it, copy/paste files through External Storage mount points is a piece of cake 
Well ... kind of ... it fails sometimes, especially with folder containing huge amount of files 
Problem:
I'd like to find the most efficient way to do that. Would you offer some suggestions?
System n.1 runs OC ver.8 hosted on a web server, which offers ftp access, but no SSH (or other console). File stored = 40GB.
System n.2 runs OC ver.10 hosted on a web server, which offers ftp access, but no SSH (or other console).
I need to copy those 40GB to the new system (copy, not move!).
Both servers are remote, I don't have physical access to them.
I have OC Admin access to both.
I used to use an extension on OC8 which let me copy file/folders but is not maintained, and has many flaws
I can mount external storages (each with its own limitations).
Being it 40GB of data, I need a reliable solutions.
I already considered downloading everything from system 1 and uploading it to system 2; but that would take way too much time, since I don't have access to a very fast data line.
Any suggestion will be appreciated