ReadyNAS OS6 VMDK download - Storage Appliance Virtual Machine?..

The GUI is only available via HTTPS, just like a real NAS.  I don't recall if the command prompt is as well (meaning you have to enable SSH via the GUI to use it, even in the local console). Make sure you've set up the Virtualbox Network settings with the proper protocol so it appears on the network you want to use for access.  I actually have one set for Host Only for access from the host computer and one for NAT Network for access from other VM's.  You can use Bridged if you want another real machine to have access or if you need to do an OS update or anything else via the Internet (I just use downloaded files to update)  It does support two network ports, I have not tried more.  Good old RAIDar will help you find it, if you need that.

The software development package is something else entirely.  It's a standard Debian desktop installation with the development tools already installed and a chroot environment for that development.  Don''t ask me any more about that.  I'm still looking for "ReadyNAS App Development for Dummies", since I'm mostly a sparky, not a softy.  I could probably make my way though creating ReadyNAS-specific versions of standard Linux apps with a little coaching or docs not written for somebody that's done Linux development for years.