REPLY: Energy Drain has the Saving Throw line "Fortitude partial; see text for Enervation" (a spell which, incidentally, does not descibe the saving throw for preventing a negative level from going permanent in it's description text), and also qualitatively has it's effect reduced when the saving throw(s) is (are) successful (from negative level(s) followed by permanent level loss, to negative level(s) followed by no permanent level loss).
To an uneducated layperson like me, that puts the spell Energy Drain in the category of attacks that Mettle protects against, specified by the following phrase: "...attack that normally would have a lesser effect on a successful save (such as a spell with a saving throw entry of Will half or Fortitude partial)...". Every successful save against the permanent level loss lessens the effect of the attack, and the attack in itself is a spell with a saving throw entry of Fortitude partial (and as such even falls within the example pointed out by the ability description).
The fact that the saving throw to avoid level loss is actually an innate property of the negative level mechanic does not change the way Mettle is phrased, nor the way the spell is written. Sure it's obviously not intended to work that way, and how it actually is intended to work is obvious, but I have seen no indication that this is a thread to discuss quirks in the intentions of the game designers.
Also, I forgot taking into account that since the saves are separate, it's not dependent on one save, but rather on 2d4 saves. Sorry.
ON TOPIC: Noone mentioned the original paradox so far? A L10 Dragon Disciple (at least before any errata) gains the dragon type, thus disqualifying the character from fulfilling the prerequisites for the class, which strips said character of it's class features, including the one that gives it the dragon type... and yeah. Liar's paradox.