Earlier quoted context omitted.
You can distribute writes over multiple disks.
Out of pure curiosity but wouldn't the number of disks required to guarantee 100% write-backup at 100% peak times be pretty big? A quick search says HDD is ~200 times slower than RAM (on average, consumer grade).
I guess it should be big indeed. To somewhat reduce its number, you can buffer writes in another DRAM if those peaks are expected to be short. You can overprovision if you have multiple clients and expecting that they won't peak write together for a prolonged periods of time.