Cost of downtime? You aren't chatting at work, I hope.
Inserting a hard drive into a computer is quick and very easy. Copying data over takes time, but can be done very easily (set it up overnight). You know ahead of time if the hard drive is going in most cases related to wearing out from use as you are mentioning, so you can leave it in and copy stuff easily without any other software needed. Or, you can ghost it and that doesn't lose data unless you don't know what you are doing. I run 4 drives in my computer and have replaced only two from dying over the past 15 or so years. My computer is online 24/7 and connected to IRC 24/7 in multiple channels (usually 5 at all times) and I log them. I have no significant hard drive deterioration caused by logging.
I personally wouldn't want to see mIRC write logs every half hour or whatever. In large channels like you mention, the cache could be considerable. Yes, text writes quickly, but it can still cause a small hiccup in CPU usage from large channel writes. That isn't a good thing when playing a game to have a game freeze for a split second every half hour. A split second may not seem like much, but in some games, it can be the difference between life and death.
