Hey! Mac newbie here. I was having the exact same problem with my Nexstar 2. I ended up having to format the harddrive is OS extended journaled for it to be recognized by my mac. Although when I plugged it back into my xp pc laptop, it was not recognized. So it turns out that the Nexstar is supported by mac but not both mac and pc at the same time. Are there any external hard drive encasements out there that supports both operating systems, or is it not a matter of the encasement but a question of a lack of communication between osx and xp?