There are plenty of situations where this might happen if they’re in your room: a lightning strike can cause a surge that causes the disks to fry, a thief might break in and steal your system, your house might burn down, an earthquake could cause your disks to crash, a flood could destroy the machines, and a sinkhole could open up and swallow your house. You may laugh at some of these as being improbable, but I have seen _all_ of these take out systems between my times in Florida (lightning, thief, sinkhole, and flood) and California (earthquake and house fire).
The fix for this is the same fix as being proposed by the parent post - putting physical space between the two systems so if one place become unavailable you still have a backup.
Here are some examples where that happened:
1. Drive manufacturer had a hardware issue affecting a certain production batch, causing failures pretty reliably after a certain number of power-on hours. A friend learned the hard way that his request to have mixed drives in his RAID array wasn’t followed.
2. AC issues showed a problem with airflow, causing one row to get enough warmer that faults were happening faster than the RAID rebuild time.
3. UPS took out a couple racks by cycling power off and on repeatedly until the hardware failed.
No, these aren’t common but they were very hard to recover from because even if some of the drives were usable you couldn’t trust them. One interesting dynamic of the public clouds are that you tend to have better bounds on the maximum outage duration, which is an interesting trade off compared to several incidents I’ve seen where the downtime stretched into weeks due to replacement delays or manual rebuild processes.
Same manufacturer, same disk space, same location, same operator, same maintenance schedule, same legal jurisdiction, same planet, you name it, and there's a common failure to match
HA! I had received new 16-bay chasis and all of the drives needed plus cold spares for each chasis. Set them up and started the RAID-5 init on a Friday. Left them running in the rack over the weekend. Returned on Monday to find multiple drives in each chasis had failed. Even with dedicated one of the 16 drives as a hot swap, the volumes would all have failed in an unrecoverable manner.
All drives were purchased at the same time, and happened to all come from a single batch from the manufacture. The manufacture confirmed this via serial numbers, and admitted they had an issue during production. All drives were replaced and at a larger volume size.
TL;DR: Drives will fail, and manufacturing issues happend. Don't buy all of your drives in an array from the same batch! It will happen. To say it won't is just pure inexeperience.
Is that comparison fair? If you have 2 raid-5 mirrored raid 5 boxes in your room and all disks fail at the same time, you should complain. And that won't happen. These entire datacenter failures should be anticipated, but to expect them is a bit too easy I think. There are plenty of hosters who don't have this stuff even once for the last decade in their only datacenter. I do not find it strange to expect or even demand that level but to protect yourself if it happens in any case if that fits your specific project and budget.
Edit; OK meant that raid-5 remark in the same context as the hosting; it can and does happen but it shouldn't; you should plan for a contingency but expect it goes far. We never had it (1000s of hard-drive, decades of hosting, millions of sites) and so we plan for it with backups; if it happens it will take some downtime but it costs next to nothing over time to do that. If we expected it, we would need to take far different measures. And we had less downtime in a decade than aws AZ had in the past months. I have a problem with the word 'expect'.