The ReadyNAS use what is called a "self signed certificate". When you run HTTPs, that certificate allows encryption, but the web browser can't verify that the NAS is really your NAS. That requires a certificate that is signed by a certificate authority (CA). The NAS isn't Netgear's - it's yours - so Netgear can't provide a signed certificate for it.
Options you have (in order of difficulty) are:
(a) just ignore it and click through
(b) attempt to install the NAS certificate as a "trusted" certificate on all your devices. Details depend on the operating system.
(c) get a CA certificate and install it on the NAS
(b) is quite a bit harder than (a). My advice is to click through the warning. A variation is to use FireFox to manage the NAS - it will let you save a security exception, so you only need to click through once.
