Redundancy is the key to securing data using backups and archives. When designing a backup system you must put on your pessimistic hat and think of all the things that could possibly go wrong with your data. You must then design a system to minimize risks at every possible failure point. One CRITICAL requirement of an archival system is multiple levels of redundancy.
The first line of defense is protecting against catastrophic data loss. These types of losses can be caused by fire, theft, or hardware failure. The best defense is to ALWAYS have your data in at least two places with one stored offsite. You can use multiple external drives with one on-site and one in safe deposit box or you can use online cloud storage. Ideally, your data should be in three places: (1) a working copy on your computers, (2) an on-site backup, and (3) and off-site archive.
Every backup and archive system MUST use multiple storage systems. I prefer external hard drives with one on-site and one off-site. When backing-up the current data on the working computer copy the data to a backup directory on the on-site drive. Then swap the on-site drive with the off-site drive. The old on-site drive is now the off-site archival drive, and vice versa. The copy those same files to the current on-site drive. Your data is now in two places with one off-site.
There are times when you will accidentally delete a file or folder. While this error can be annoying you should have your data well protected. When you backup your working computer I HIGHLY recommend using rotating backup folders. When it is time to backup your data create a new folder named _backup.2013-02-03 and copy all of your data into that folder. After several backups your drive will contain multiple sets of backups. You will want to keep multiple backup directories on the drives. If you need to access a previous backup then you have it readily available. You should maintain 3-5 backup sets. When you have too many backup sets then backup your current data and erase the oldest backup directory.