Thanks whiteb for making me remember the 2 Gb size limitation!! I read about it time ago and I had completely forgotten it !!!
But the problem is only half solved

I've reinstalled OS 3.1 and the installation finished OK, no error messages about lack of hard disk space and OS 3.1 runs fine.
But when it comes to install OS 3.9 and I double click the "Install" button, the error continues :
c/Installer
Program Failed (error #80000006)
Wait for disk activity to finish
Sometimes the error is not only about "c/Installer" :
WBL
Program Failed (error #80000004)
Wait for disk activity to finish
I think the problems are related to the Elbox FastATA controller (is the only thing I've changed since my last successful OS 3.9 installation).
Any idea on how to isolate the problem ??
Regards,
Manel.