If the hardware failure is due to a manufacturing problem, then it affects every machine. If it's problem with the production batch, then again, unless you bought from different batches, you haven't reduced risk much.

And if it's a software problem, then again the redundancy isn't helping you.

Basically you haven't succeeded in getting 5 independent tries in the problem -- the failures are highly correlated.

It's the same reasoning that led to the financial crisis -- you have a CDO or an MBS that has a bunch of different obligations so you think you've diversified away risk, but in actuality there is one big factor that causes everything to fail at once.

Also - same shipping way. If I buy four disks from the same shop, then those four disks came in the same delivery to the shop (so they were all dropped at the same time), are handled by the same picker, thrown into the same parcel and dropped a couple times the same way.