Daniel, thank you for your answer.
I have tried to format my usb key 64gb with fat32 system. But windows does not allow that.
I found a little software to format 64gb usb key with fat32 and I tried to copy all the files from my iso file. But the problem is that one file (install.esd) is bigger than 4 Gb which fat32 does not allow.
Are you sure that this has to be formated with fat32 ?
(update August 7th : I finally managed it by downloading another iso with smaller files. )