Storage needs are always growing,
Look for a External Case that holds 2 Hard drives, if you put 2 2TB HD's in there you have a 4TB Volume in one case. Double it and you have a backup. as you need more, you just replace with bigger hard drives over the next year or two as your need surpass the current setup
Even if you have a Drobo/unraid, you dont have a backup, you have some safety in hard drive failure. but not data failure !!!!!!!
I'm no data storage or backup expert, but I assume you mean data corruption. That is definately a problem with any system like RAID or unRAID. Pictures, Home Movies, and important documents get backup up regularly to 3rd and 4th locations. That is definately a negative to a redundant storage system. I use my unRAID system for mostly movies and TV shows. If one gets corrupted, I'm not concerned. It is easily replaced. A drive failure scares me so much more because it equates to hours of re-ripping.