Hard disk woes

I’m starting to think that Maxtor disks aren’t as good as I thought they were. I bought 3 about 10 months ago, shortly followed by another one. All the same model and size, and created a linux software raid RAID5 array out of them.

All worked fine until about a month ago when one of them failed and because I’d never swapped a disk with linux raid, I backed up all the data on the array before taking it down. It was lucky I did as I nearly lost it all at one point whilst trying to fix the array by adding a fifth disk (newly bought).
Just as I receive the replacement disk for the one I RMA’d, I find that another disk has gone in the array and now I’ve got to go through the whole thing again.
Overheating machine?
Bad luck?
Faulty batch of disks?
Maxtor disks just aren’t any good?
Probably a mixture of the last 4 as I have drive coolers installed and they seem to be working.