40GB is less than 10 DVDs worth of data. (less than 5 for dual-layer). With a little bit of up-front hardware cost, you can back up your collection, and take it off-site.

What you're discussing isn't quite as important with modern hard drives -- they're much better about checking for bad sectors, and the mean time between failures is much longer than the days of ~40MB disks. If you really wanted redundancy, you'd also want to deal with the failure of the disk controller, not just individual sectors, which would require moving to a RAID solution (or at the very least, mirroring your changes to another disk)

So, to answer your questions:

• 1. It depends on the hard drive mechanism. You'll have to look at the manufacturer's information.
• 2. Make sure there's a copy of the data elsewhere. (and if not a complete copy, parity information ... ie, something that can be used to recreate the data, such as what you're talking about)
• 3. If there is, I'd expect it to be in CPAN, and I'm not seeing it. There are modules if you search for ECC or CRC.

(yes, I know you can't rebuild the data from a CRC, but it would give enough to sense if something's gone wrong)