I've never heard of anyone doing that for a home nas. I have one and I don't keep spare drives purely because it's hard to justify the expense.
I did end up with a spare before at the 3 year mark but the bathtub curve of failure has held true and now that so-called spare is 6 years old, unused, too small of a drive, and so never planned to be used in any way.
The conventional wisdom is that you should not store drives that don't get spun up infrequently, so what does it mean to have spares unless you are spinning them up once a month and expecting them to last any longer once actually used?
I'm not made of money. I just don't want to make excuses over some $90 bit of junk. So I have have spare wifi, headset, ATX PSU, input devices, and a low cost "lab" PSU to replace any dead wallwart. That last one was a life saver: the SMPS for my ISPs "business class" router died one day, so I cut and stripped the wires, set the volts+amps and powered it that way for a few days while they shipped a replacement.
i budget 300usd each, for 2 or 3 drivers. that is always the sweet spot since forever. get the largest enterprise model for exactly that price.
that was 2tb 10yrs ago. 10tb 5yrs ago.
so 5yrs ago i rebuilt storage on those 10tb drivers but only using 2tb volumes (coulda be 5, but i was still keeping the last gen size as data haven't grow), now my old drivers are spares/monthly off-machine copies. i used one when getting a warranty for a failed new 10tb one btw.
now i can get 20tb drivers for that price, i will probably still only increase the volumes to 10tb at most and have two spares.
