Not on my watch!.. 
That is not quite true. It depends on your NAS, maybe not for Western Digital; but you certainly can for ReadyNAS. I have replaced failed 500 gb drivers with 2 TB ones.
It depends on the type of RAID I think so, due to the way it distributes the parity bits, is very fussy in this respect and others not so much…
Comparison of ds218+/ds218/ds218play
https://www.synology.com/en-global/products/compare/DS218+/DS218/DS218play
I have ds218play which has worked quite well for storing and serving media content.
Raid 5 is not possible to implement with 2 disks as it requires 3 at minimum.
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You might want to have a browse around this site - Span.com. There are a lot of videos comparing individual models, different brands etc etc. In a way that’s quite entertaining too - no glossy sales stuff, just facts and what appear to be honestetly expressed opinions.
I have no affiliation with them at all, I haven’t even bought anything from them yet (so can’t comment on their customer service). They’ll be my first port of call when I do need to upgrade ny Synology though.
Thank you for the recommendation.