I am freaking out over here! I upgraded to Mountain Lion and everything seems to work ok except the Mail app. When I try to open it, I get this error: I have my Mail folder on an external hard drive, not on the boot drive, and this worked fine in Lion. Permissions are fine, even though the error says they are not. I tried to reset them, did the Command+R and the resetting permissions and ACL from there. Did a bunch of searching online and found on link that said alias no longer work in ML and you have to do syslinks so I tried messing with that and somehow deleted the whole Mail folder, all 44GB of it! Luckily I have a clone backup (though I probably lost all the mail from today?) and I have a Time Machine backup as well. Right now I'm waiting while it copies over the Mail folder from the cloned backup onto the external drive again. What a mess.