People often implement some sort of redundancy or backup and think their data is safe. That is incorrect.
To correctly protect your data you must list all the possible ways in which data can be lost, and then address each and every one of them (or the ones you think are pertinent)
Here are a list of common disasters and the ways to protect against them:
1. Fire - offsite backup, DO NOT use a firesafe as they are meant for paper documents (350 internal temp max... which will ruin your data, but not your paper documents)
2. Flood - offsite backup, waterproof media?
3. Swatting: Swatting comes from the term SWAT, SWATting is when someone places an anonymous call to the authorities, and lies, claiming that you traffic in illegal materials such as illegal gun trading or CP.
Even if you are completely innocent, your computers and drives will be held as evidence for years afterwards. Offsite backup is the only protection against this (as it will surely cause your business to fail otherwise). Note that this isn't advice to circumvent the law, if you are guilty they will likely go after your offsite backups as well. But if you are innocent this can save you. (I have read stories of people who run small software business from home having that happen to them, there was never any evidence found of any wrongdoing on their part, but the FBI kept their computers and data)
4. bit rot - use a checksumming filesystem (only ZFS at the moment); create par2 recovery records.
5. Cosmic rays bit flipping - same as bit rot
6. Drive failure - use redundancy (raid 1,5,6, 10/1+0/01/0+1 etc); or backup. Note that neither RAID 0 nor JBOD are redundant and will not help.
7. Controller failure - use a scheme that allows you to replace the controller (aka, NOT a motherboard raid controller... motherboard RAID1 is ok though, raid1 is fully portable even on mobo controllers).
8. A controller failure with a controller no longer available on the market - use a system that allows you to switch to other controllers (aka, OS based software raid like in linux, solaris, windows server, or open nas; or use controllers from a company that historically makes newer versions support older controller made arrays).
9. Theft - offsite backup OR/AND worthless media backups (DVD-R). they will take your fileserver, iphone, external drive, and flash drives. but unlikely to take your DVD-R collection.
10. Virus - NOD32 antivirus , but ideally, have an external backup that you turn off / unplug except for when you manually connect it / turn it on to perform a backup. (not that i have seen a virus that just deletes data).
11. Drive erroring (due to a bad drive or PSU) but still writing - use a checksumming filesystem (only ZFS at the moment) AND redundancy (RAID1, 5, 6, etc). using only RAID will leave you with corrupt data as it has no way of knowing which copy of readable data is corrupt and which is correct. Using only checksumming filesystem will let you know it is corrupt, but without a non corrupt copy to recover from.
12. Sabotage - offsite rolling backup; internal security.
13. Lightening strike - backup to optical media (CD/DVD/Bluray), or backup to an external drive that you physically unplug from both the power AND the PC when not in use AND doesn't sit on a metal surface. If it is sitting on the PC case, or if it is plugged into the PC via a USB cord, that is enough to fry it!
Also, use surge protectors on your PC / Server / External drive (not as safe as unplugging it, but it might save your data and hardware)
Also, please note that whatever method of data protection you use, it is CRITICAL that you:
1. Check to see that the backups are happennig as expected. (is it really backing up daily? or did a careless employee disable it 9 months ago and did not turn it back on later? is the external HDD it is backing up to even powered on? ps. both are real stories i dealt with)
2. Check that the backups are readable.
3. Check that all needed files are being backed up (for example, your firefox bookmarks might not be backed up because you can't copy them while FF is running, as they are considered in use)
Terms explained:
Rolling Backup: Rolling backup is what you have multiple backups from different time periods. For example, you have a complete backup of all your files from 1 week ago, 1 month ago, 2 months ago, 3 months ago, and 1 year ago.
PAR2:
http://en.wikipedia.org/wiki/Par2
ZFS:
http://en.wikipedia.org/wiki/ZFS