In reply to The Lemming:
Synology were my favourite NAS maker but I used to use CloudStation a lot for syncing a number of computers, phones and tablets. Unfortunately Synology won't allow you to save to an SD card on Android after a certain version, so if f*cked everything up, which is a real pain, because it all worked great.
Despite lots of people moaning on the Syno. website, it doesn't appear as if they're going to change anything, it's been an issue for 3-4 years.
There's a work around using other software on the NAS but it's an arse about.
AAAaaaanyway, I think there kit is great, problem above notwithstanding and you can go to a 4 drive and stick your two drives in with other drives and it should all work straight off.