SOLVED!!!!
I found a way to solve the problem! I post the instruction for other Windows owners with the same difficulty.
1) Download a freeware utility called Bootice at
http://bootice.narod.ru/.
It's extremely important to download BOOTICE v0.7.8.0 not a newer version otherwise it won't work.
2) Follow carefully the instruction present here
http://www.pendrivelinux.com/restori...key-partition/
Important: don't use bootice download link from this page because it leads to a newer version of the program while you need to use only v0.7.8.0 . In the article is mentioned that Windows users can also use HP USB Format Tool
I've tried it one hundred times and it doesn't work!
3) After the format reinstall the firmware.
With this procedure my Opus is brand new again!!
PS: This has been tested only on Opus with firmware 2.1.