The storage requirements are kind of a seperate issue depending on where users are homed... if you're installing into a site with an existing Exchange network and users are homed off box, Unity doesn't need much HD space, if you're a POV install, you need to make sure you beef up your storage space.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.