Thanks for the replies everyone , much appreciated .
I was trying to save on a second hdd for the NAS if single hdd could mirror the storage one in my pc but sounds like I should invest in the second for the NAS ;)
There are ways to kind of mirror from an internal drive to some sort of external drive. In the case of a NAS, that "external" drive would be a network drive. So, it would be more a matter of either doing a form of a duplicate backup or file sync using a third party program. Generally, this would not be a "live" sync/mirror (i.e. happening instantaneously). It would happen on some backup/sync schedule that runs periodically. If you want an NAS and "true" mirroring, then your best bet is a RAID NAS with at least two bays (and a drive in each bay).
If memory serves, Windows 7 does have a built-in Soft RAID function that would allow you to mirror from the internal drive to an external USB drive...I believe. You would likely need to post that question to the Windows 7 forum most likely.
If I do this shortly will it be fine swapping to a new pc when Win 10 is released later this year ?
If you go with the NAS, then you should have no problem as a NAS is a network drive. You just mount your "shares" (aka "network drives") from the NAS to your new computer.
The only possible problem is if you pick a cheap NAS whose manufacturer decides not upgrade the NAS OS/firmware to work with Windows 10 (assuming it does not work with no modification). I had this happen to me with my first dual drive RAID 1 NAS. It was a LaCie RAID NAS that I used with my Macs. When a when Lion (Mac OS X 10.7) came out (I believe it was that OS version), they decided to no longer support it and not upgarde the NAS' firmware/OS to be compatible with Lion even though that NAS was released like 3 or 4 years before. As a result, I don't buy LaCie products anymore. I have not had the problem with the ReadyNAS line.