I was afraid someone would say something like that.
So basically it is all random and to reduce the randomness get an extra HDD which is used exclusively as backup, not intermittent use...
You think SSD or something new will change this?
Also which brand is the most reliable, Western Digital, Samsung, Hitachi, Seagate..?
Backup is not simply because of hardware failure. Even if you had an "invincible" drive there are many thing that could still kill your data. Accidental deletion, OS failure, viruses, computer got stolen, etc. My daughter at two years old once pulled pulled the plug while one of my servers was doing a disk check. Boom, data gone other than backups, no hardware failure.
I've been a system administrator for nearly 20 years and lost data due to human error far outweighs hardware failure. Even if drives never died I would still be just a vigilant at backing up.
I think SSDs will improve hardware reliability in the long run. I've had good luck with them, but there are of course lots of horror stories too. They do have a big downside in that they can truly erase data in normal operation. TRIM can erase data permanently right away, while a mechanical drive keeps data unless it is explicitly overwritten.
All manufactures have good and bad drives. None is really better than the other, now that Maxtor is long gone (they were bad). If you want to be really safe buy a different brand of drive for your backups. My main array is currently Seagates with a primary backup on WD drives.