I'm talking about any problem that a hard drive could have. One drive dies, and the response to that drive dying is to go beat the shit out of the remaining dr

I'm talking about any problem that a hard drive could have. One drive dies, and the response to that drive dying is to go beat the shit out of the remaining drives, and given their large size, that beating goes on for days (possibly). Yes, an unrecoverable read can be one failure mode, although not necessarily the scariest - you can often tell your raid hardware to ignore UREs and just rebuild anyway, hoping that the missing bits aren't crucial (which in the case of photo storage isn't that bad of a bet...).