Bloomin’ hard disks

How can it be that a disk runs just fine, with no problems or reported errors, but when you add another disk into the system, set up software RAID1 and raidhotadd that the first disk, the one thats been fine for ages but you want to just b sensible about, how is it that the bastard disk can fail just at that moment and refuse to allow you to get any data off of it? Hmm? Arse.