hard drive upgrade (replace existing)

Something to keep in mind regarding firmware release and online expansion. It's not just a question of what release you have installed, but what version was installed when the X-RAID volume was originally created. In particular, if you were running RAIDiator 3.x (or earlier) when initially installing your ReadyNAS and then later upgraded to 4.x... you do NOT automatically get the benefit of some of the volume changes that 4.x supported -- block size and online expansion.

To get the extra benefits, you would have to perform a FACTORY RESET (completely wiping and re-creating the X-RAID volume, and requiring you to restore your data) in order to accomplish that.

I lucked out, I guess, and 4.x happened very soon after I got my NV+ in 12/2007. When the time came for me to expand (4x500 to 4x1.5T), I did have to do a few restarts but the NV+ otherwise was available for use during the entire process.

I agree with the frustration that this is not well documented in product manuals or very visible in the web site... but if you search the forums regarding online expansion you will see the discussion of this topic come up repeatedly.