First I want to thanks everybody for your help and reactivity.
Then, I've already added users and PW, butr nothing works.
As you said, I use both WINSCP and putty. WINSCP for the windows interface and putty as seems to work better than WINSCP.
I may use these SW, if OK to transfert files to my server from an external PC. I'll search on the web (I'm a newbie and not sure that these softs do the job from external PC ?).
Anyway, I must reinstall my USB drive. All works fine, as the HTTP (and PHP) server for now. But, working on the FTP conf files, I've made something (what ?) that lead the plug to lost the SSH communication. Then never communicate again, when boot on the USB. Still works fine when booting on the plug itself. So I've to re-format the USB drive and reinstall everything.
TBC...