Well, folks: I finally found out what went wrong, and I'll try to explain it as good as my ignorance of Apple hardware, OS-X software and firmware, and computers in general, allows me.

But first things first: I have good news for you: if you ever get the white screen of death, don't panic! And, unless still under guarantee, don't take your computer to an Apple centre for them to steal you yet more money by making you pay for a new logic board and/or a new hard drive. Probably you can solve the problem by yourself, spending only a few -very well invested- dollars in a pentalobed or a phillips #00 screwdriver, plus maybe a Torx T5 one.

Pleae correct me if I'm wrong, but, as far as I've guessed and investigated, it works like this: your Macbook Air's firmware (probably also any other Macbook, but I can't be sure about that) is always instructed to try to, in the first place, boot off certain piece of hardware: be it the hard disk, an external DVD unit, the install USB stick, the network, or whatever. This option can be set in the OS's Startup Manager. When the actual boot setting is any other than the hard drive, then, at boot time, the firmware will look for the instructed medium to boot off, and if for whatever reason it doesn't find it, then it will try to boot off the hard drive as a "last resource". BUT when the actual boot setting is "hard drive" and, for whatever reason, the data is corrupted or something is wrong with the boot sector, then the computer will NOT try to find any alternative device, or a network, where to boot off: it will keep forever trying to boot off the hard drive, and thus you get the dreaded white screen of death. The Apple engineers are so smart!!

But it's actually not dead: neither your motherboard nor your drive are dead. They're just in deep coma.

How can you wake it up from this coma? Simple: remove the back cover and disconnect the hard drive. Believe me: with the right screwdriver, it's very easy to do and even you can do it. Then, and only then, if there is NO hard drive where to try to boot off, the firmware is programmed to look for some other medium: USB stick, external DVD, network... So, plug this medium in and... voilá! There you get your little spinning animated icon and, eventually, the Apple's apple; the startup thisk will boot and there you are anew to life.

Now, once you've managed to boot, the FIRST thing you have to do is go to the Startup Manager and set the firmware to boot off your whatever startup medium is (otherwise, upon reboot you'll be in the same S*ITuation). After this, you can plug back in your hard drive and reboot your computer. Hopefully it will boot off as instructed (the USB/DVD/Network), it will recognize your hard disk, and you can use the Disk Utility to repair it, reformat it, or whatever you want to do with it. After this, everything will go back to normal, and you've saved the Apple care center's $500 bill.

Hope this will help someone to avoid the three weeks anguish I've gone through... I'm sorry to say that it is A SHAME on Apple's part to sell such cr*p and not respond for it, nor inform about it in their support webpage, but joyfully get your money when you take your laptop to repair.

Welcome any correction or precision to this post.