What is a quartercrack?
It's a minor crack on the hoof of the horse. In terms of human injuries, it's somewhere between a hangnail and a bad foot blister.
Is it life threatening?
Absolutely not. Quartercracks are not uncommon and are wholly unrelated to lamintitis (what felled Barbaro and Secretariat). Again, it's like a foot blister--bandage it up and/or use that area a little less and it'll go away.
Is this a surprise?
1000% no. Big Brown missed significant training time last fall and winter with other hoof problems, none of which were life- or career-threatening, but all of which interrupt training. Many people thought that the quick spacing of races in the Triple Crown sequence may exacerbate his propensity for achy feet; this seems to have been right.
Does this alter his schedule at all?
If this were pretty much any other horse, odds are s/he would get a few weeks out to let the crack heal and point for a later race. But this is a Triple crown bid. He'll miss a few days or a week of training and be back to his usual routine by mid- to late-week.
Will this effect him in the Belmont?
Ah, the key question. His trainer Dick Dutrow insists it'll make no difference. The owners are saying the same thing and his vet is talking a big game. Still, it's worth looking at a pair of similar recent instances of favorites with hoof issues:
2003: Empire Maker is the 2-1 Derby favorite off two strong Grade I wins, including a measured win over Funny Cide in the Wood. In the week preceding the Derby, he develops a foot bruise, missed some training, and trainer Bobby Frankel deems him "fine". On the big day, Empire Maker runs 2nd in the Derby, barely beating the inferior Peace Rules and fails to threaten the winning Funny Cide. Was it the hoof? Maybe. 5 weeks later, Empire Maker, fully healed, returns in the Belmont and beats Funny Cide convincingly.
1996: The extremely talented Unbridled's Song enters the Derby as a stark favorite with myriad foot issues that cause him to miss training time. He runs fantastically for 9 furlongs, only to run out of steam by the eighth pole and finish fifth. While some would argue he was questionable at the distance anyway and trainer Jim Ryerson was in over his head, it's pretty clear he was not fully amped and able to win because of his feet.
So where does this leave Big Brown? While it doesn't improve his chances to have a minor injury, it's overstating its severity to say it makes winning impossible. Observe how much training he misses carefully, and not just jogging (like he did this morning). If he's not galloping 100% by the weekend, it's then time for the connections of Denis of Cork to get excited. In the meantime, here are our revised antepost odds:
Denis of Cork......4-1
Tale of Ekati.......15-1