Thanks to everybody for yours replies.
I've found that the problem is about the mk_mx28_sd/mk_hdr.sh scripts or about my host because using an Adeneo's virtual machine (Ubuntu 10.10) it works fine.(In this machine, the two Adeneo's scripts are not the same of the attachment, see below).
So my board it's not broken, it's a problem of my host.
May I ask you what machine is better to use to setup the host for the i.mx28 demo board?
Now I use an Ubuntu 10.04 LTS 64Bits.
In the documentation Freescale certificates Ubuntu 9.04 and 9.10 but now they're not supported already.
Where can I find the right mk_mx28_sd/mk_hdr.sh scripts?
Following Daiane Angiolini's link I've found this scripts(see attchments).
Are they correct?
Also are they correct just with some Linux distros or with every distros?
Thanks to everybody and congratulations for this active community and it works.
Alessandro