What Discs For a QNAP TS212

I would go for WD RED drives.

Yes , Red is the way .

+1 for WD RED

The blurb sez .....  WD have taken & improved on all the best features of green & other WD drives,  they are designed for home & small office NAS 1 to 5 bay units from makes such as QNAP & Synology. They are notably quite as they run usually slower than many other high performance disks. The Red drives slow down when not under heavy demand, allowing the large cache to feed the data steadily.  They are ideal for streaming. 

Every serious NAS company has in the site of every NAS model the certified HDD's. Said that, from about 1 months I have a Qnap HS-251 with 1 WD red 4Tb (bought new with te NAS) and an older WD green of 3Tb that was lying around and that I didn't use any more.

So I programmed the NAS to "see" 2 single discs. The WD red contains the "public folders" for streaming and the WD green is "hidden". The NAS is programmed to make a backup from the red to the green HDD on daily base and everything is working flawlessly (be aware of the fact that green HDD's are not suggested for NAS !)

When I will reach the full capacity of the green HDD I will buy a new 4tb red HDD and will program the 2 bays in Raid 1 ! But maybe when I will reach 3tb's in music SSD's will have reached 6tb capacity and will cost 50 Euros !

Is it possible (or wise) to just change one drive with a different make/capacity, or best to replace like with like or change both at the same time

Best option? Cheap backup (via USB drive or 2nd cheap, noisy NAS), no RAID, large SSDs–less heat / power consumption / noise, and fewer illusions that regular backup is not necessary.

If your NAS cannot accommodate 2.5" drives (another option BTW), the Icy Dock drive adapters are inexpensive and work very well to adapt a 3.5" drive bay to 2.5" (notebook) size.

+1