Seems that my crystal ball misinterpreted your information so far. The words "newly connected" and "automatically mounts" apparently triggered the "I want to mount my USB pen drive in a different directory" chain.

As long as your external RAID system is connected using the same device path, just add it in /etc/fstab (and I'm sure OSX has a nifty GUI for that) to keep it there even after a reboot.