I had an instance a few years ago where one of the WD Reds populating my original NAS failed.
At the time I had also decided not to bother keeping a hot or cold spare at home after roughly calculating the odds of ever needing an urgent replacement vs the additional expense. (After all, the chances of a simultaneous double failure or two drives failing within say, 48 hours, are pretty remote (but obviously not impossible).)
As with many others I decided to simply order any replacements I may need on an as required basis. However, when my priority next day replacement arrived it was defective, (which then required a ridiculously lengthy RMA process under the circumstances). While that RMA was still going through I paid out for a second next day replacement & that too was faulty on arrival!! (I order important components like HDD's from a well known UK retailer & I certainly wouldn't Amazon Prime such components or you're literally asking for trouble with the way Amazon package things or their delivery agents throw things around
.) Eventually, a third drive arrived which functioned normally & I was able to successfully rebuild the volume, thankfully with no harm done at all. (I was also eventually reimbursed for the two earlier faulty replacements but that took nearly two weeks to go through as well & I had to essentially pay out for three drives in three days then wait to be reimbursed.)
Lesson learned!!! If a second drive had failed outright or the volume degraded on a second drive in the time I was messing around waiting for a functioning replacement, it would have been a whole other story. (Obviously, I backup any vital/important data which all fits on to external HDD's, but backing up TB's of multimedia is another matter entirely & ultimately very expensive to purchase that level of redundancy for a typical home user.)
Ever since that incident I kept a cold spare just in case & I picked up said spare at nearly half the normal price on a snap deal anyway, so it's definitely worth having IMHO. I've also since acquired additional spares after upgrading another NAS too. (It's the completely random & unforeseen problems that invariably end up biting you squarely on the backside with this type of scenario
.). I recently had another older drive fail in another NAS & thankfully I was able to put a spare straight in to rebuild the volume immediately without several days of sweating
.
Granted, it's not cheap to just buy a spare to sit in a draw/on a shelf that ultimately may never be required anyway, but how much is all the data that you can't/don't backup from the NAS or have additional redundancy for ultimately worth to you should the "
improbable" actually happen?
RAID/SHR gives you a degree of redundancy & essentially buys you time if you act swiftly, but, it's those "what ifs" that'll get ya
.
Entirely subjective which way you go on this. It's the added peace of mind a spare gives you should something you just can't control suddenly crop up really
.
Enjoy.