23 and me said I was a match on my maternal side to a 1/2 brother - we were both adopted to different families and are trying to find out which side we've matched from. I'm U5b2a2 and he is X2c1 - possible??
If we suppose that there is no error in mtDNA mito-group detection, it means (and means ONLY) that you have no common direct maternal ancestor. I.e. you have no common mother, no common maternal grand-mother and so on.
It does not mean that you are not related. I can suppose that your half-brother is in fact the son of brother of your mother. Or something similar. It is difficult to say because you gave no useful information at all. If you share with us a little bit more info like the total shared DNA in cM and the distribution of common segments over the chromosomes, we will be able to make more reliable hypothesis about your relationship.
Also I want to advertise you to download your raw autosomal data to GEDMATCH service. It is free and has more interesting tools to confirm (or dissaprove) relationship. Another idea is to upload your data to competitor lab - FTDNA. The basic transfer is free and gives you access to their match database. If you convince your half-brother to do the same it will be great!