Thank you for your answer.
As I'm not a unix user, I followed your link for the livecd, and booted the computer with this one. I also disabled the automount (unchecked 2 options, and checked a third one) as described in a link found somewhere in the thread.
I connected the device without battery/sdcard/sim card, directly on the computer (without hub) that has standard usb2 ports (not usb3).
The only thing I'm not sure is the unity, but I suppose the given link in the first post is the good version.
now I'm letting it charge for the night on a wall charger, because I've seen somewhere that if sdX does not appear, it can be because the device is not bricked but battery low (I starterd the update at 87% battery), and someone succeeded this way, also bricked with the 'fastboot oem write... 3' command.
Also, please note that I'm using the GSM version, NOT the CDMA.
Thank you for reading and helping.