My wife and I back up our pictures, etc. Mot only on an external hard drive but also on CD/DVDs, and a thumbdrive. One can't be careful enough.
I back-up monthly on DVDs, backup daily with Time Machine to an external hard drive, and have backed up with Blackblaze online.
I download to Desktop, Fix the Meta-Data, Then export to my LaCie 1-T GB, Burn, later to DVD, Let my Time Machine Pick up on the other external Hard Disk. I pick out what I want to print, put them in a folder, and send the finish photos to about three difference folder for best photo of year, Contest Photos, and Photo date photos.
I can't choose just one because I backup my photos in several different ways. I burn CDs and I backup to an external hard drive and the good ones are also backed up on a thumb drive.
For my needs, two independent hard drives plus my computer's HD should suffice. All three back-up methods have pros and cons. Using all 3 methods is the ultra hard core startegy without a doubt.
I use the WD Studio 2TB drives which include mirrored back up.
Then I take the external hard drive off-site and rotate with another one so I always have a copy of my most recent work off-site.
I keep 90% of images on external drives and then use Microsoft Sync Toy to backup working drives to a dedicated 1 Terabyte External Backup drive. Periodically I pull the backup drive and store it replacing it with a fresh backup drive.
Because I have important business files, I have 3 external 1 TB hard drives for backup: One uses Apple "Time Machine" to easily recover files. Two are using "Super Duper" which makes a bootable copy. One is always attached to the computer for daily backup and the other is in my bank lock box, updated monthly. Nothing less will do!