Want it in English?
The thing that keeps a NORMAL Xbox HD from being connected to ANY OTHER device is "ATA locking". The "lock" is something that is built into the ATA technology.
(ATA is how the HD, DVD ROM, and mobo communicate)
What happens is the HD and your motherboard's EEPROM have a Locking Key that only matches eachother. Ergo, you take the Xbox HD, put it in another Xbox, and the mobo of COURSE has the wrong key for the lock, and is UNABLE to access the HD.
NORMALLY if you have a working modded box, you can unlock it, (ONLY from the original box) you can backup the mobo's EEPROM, OR you use "Config Magic Final" to re-lock the drive with a key that YOU specify.
Now, IF you had gotten the original to work, you could have backed up the EEPROM to your X3, set the X3 into the new box, and Presto! You could have unlocked it.
What was suggested above is A only for someone who is TRULY desperate, and MUST have the data from the drive.
You see, it involves DESOLDERING the EEPROM from your original mobo, and the new mobo, THEN RESOLDERING the original EEPROM into the NEW Xbox.
I don't mean to flame, but understand, having toasted your last box I would say FORGET about it.
Consider, your Xbox live account can be rebuilt, simply go through the on-screen options, and if it comes down to it talk to a friend who has basically the same saves as you and use a mem card to transfer his saves to your machine.
I'm really sorry about the luck. IF your data is so important that you must have it, Llama can repair your old Xbox. They are the official repair site listed on the main page.
http://www.llamma.com/