Maildirs are always in the users' home directories, so all you'd have to do is create the home directories on the second HDD.
Thanks Sorry, i use IspConfig, and my question is how i can do it in IspConfig. how i can change the default path to maildirs on ispconfig
i yet know how do it. just is necesary change the value of document root, into genaral configuration of ispconfig. Thanks a lot. And sorry for my english, i am from mexico city.