Hot swap for drives is a must on a NAS. If you have to power it down to swap out a drive there is a chance that your small problem becomes a larger one. Better to replace the drive immediately and have the NAS do the rebuild without a powercycle.
If you're worried the hard drives won't spin back up, I'd say you should instead spin them down regularly so you know that risk is basically zero. If you're worried the power supply will explode and surge into the drives when you turn it on, you should not be using that power supply at all. Any other risks to powering it down?
And for the particular issue of replacing a failed drive and not wanting to open up the case while it's powered, you can get a single drive USB enclosure to "hot swap" for $20. And if you use hard drives you should already have one of those laying around, imo.
Agree, you should consider replacing your drives on your primary server (backup servers we can debate) as soon as you start seeing the first SMART problems, like bad sectors. If you do regular data scrubbing, and none of these problems show up on the other drives, I'd argue the risk that they fail simultaneously is fairly low.
Hot swap drives are necessary on data centers where you don't want to have to pull the whole server and open the top cover just to replace a disk.
But on a home NAS? What problem would having to power it down and power it on for drive replacement create? You're going to resync the array anyways.
I don't mind them and I do use them but I consider them a very small QOL improvement. I don't really replace my disks all that often. And now that you can get 30TB enterprise samsung SSDs for 2k, two of those babies in raid 1 + an optane cache gives you extremely fast and reliable storage in a very small footprint.
No, I've seen this happen on larger arrays. The restart with a degraded array risks another drive not coming up and then you are on very thin ice. Powercycles are usually benign but they don't have to be and on an array there is a fair chance that all of the drives are equally old and if one dies there may be another that is marginal but still working. Statistically unlikely but I have actually seen this in practice so I'm a bit wary of it. The larger the array the bigger the chance. This + the risk of controller failure is why my backup box is using software RAID 6. It definitely isn't the fastest but it has the lowest chance of ever losing the whole thing. I've seen a hardware raid controller fail as well and that was a real problem. For one it was next to impossible to find a replacement and for another when the replacement finally arrived it would not recognize the drives.
