I've had plenty of unreadable DVDs, but never had an offline hard drive fail.

I think blu-ray/dvd needs to be verified after creation to be safe. I wonder if 25gb can be burned and verified overnight.

By my calculation duplicate hard drives would still be less expensive than blu-ray. Although I don't know what blu-ray costs in bulk.
I assume there are bad burns like cd/dvd.

My software does a verification by default. And yes, I generally run it overnight. Or at least as I am leaving work. I generally put data on bluray and not videos so the verification process is important to me.

I've never had a bad bluray burn. The burn process is more robust than that for CD/DVD. With built in correction for bad spots on discs, etc. I don't buy my BluRay in bulk, but current pricing for my order next week is $2.60 per disk. I am now seeing 50GB disks for under $20 each. My last order of these was $27.99 around Christmas, and my first order was $47.99 June '08. I'd say that a price cut of more than half in 14 months is pretty fair progress.