Have recently bought a new NAS – a Netgear ReadyNAS Duo, which gives me 2 x 2TB drives, mirrored to each other (so I have 2TB storage).
I set my old NAS (a Western Digital WorldBook) to copying across everything overnight. Have just checked and everything has copied fine. The ReadyNAS tells me that with everything I have copied across, I am using 8% of the drive capacity.
I have one of those too. Trying to mirror the whole of a disk that wasn’t booting as a disk image filled a lot more than 8% of it….
I’ve installed a wiki on it, but not actually used it as anything other than a plain NAS yet.