macosxhints – Enable multiple home folders on different disks
It may be obvious to the Unix buffs out there but it was new to me: you can add multiple values to the home property in Netinfo Manager. Open Netinfo Manager, under users, find your username, then select the home property and add a new value, for example, /Volumes/external_disk/users/me, with everything after /Volumes/ being the path to your home folder on an external disk. Now the first value points to a home folder on an external disk, the second value points to a home folder on your internal disk.