Their free version is only for 3 total drives (from the website):
For those building their own server, unRAID Server OS is available as a free download.
Unregistered unRAID Server OS supports Basic functionality:
up to 3 hard drives
all features except User level security
To support more hard drives and User level security, you must purchase a Registration Key:
unRAID Server Plus key - support up to 6 hard drives
unRAID Server Pro key - support up to 16 hard...