Good and Bad Strategies for a home NAS
I was sad to note a valued colleague lost a whole bunch of data this week due to a NAS failure. I haven't wanted to bother them to get the details but I am going to assume that "lost" means that the NAS in question was not being used as a backup server, but as a file store without a backup.
This, with deference to my peer, is what I would regrettably have to call a bad strategy (no doubt they are kicking themselves for this today, and all sympathy do I send over it). While a NAS as a central file and media server provides huge gobs of data storage that you don't have to mirror to your devices, it is just one copy of what may be very dear and valuable pictures and video or worse, things you had under development for customers.
While most NAS systems use a flavor of RAID, and you should be able to hot-swap drives as they fail and keep on chugging without loss of data, depending on the type of failure, you could be out in the cold even with RAID. Let us remember that RAID, in all it's varieties, is simply a strategy for ensuring data is stored in duplicate across multiple devices. The use cases for this range from resiliency to speedier access to data. Depending on which RAID type your NAS uses, you might not have very good resiliency.
Take a look at the different levels as outline on wikipedia. http://en.wikipedia.org/wiki/Standard_RAID_levels
The image accompanying this post is RAID 0, which is commonly used. In this configuration, with only two disks, if one fails, you will lose half of your data (approximately) and the overall array is unrecoverable.
So what is a good strategy? Firstly, use your NAS as backup for your devices, not a replacement for their storage. Yes, duplicate data, but for resiliency, that's what you want. What this buys you is peace of mind and data sharing in your home. If you want to, you can still use the NAS as a media server, just make sure that what you have on it is mirrored to various devices or another NAS. Your 4TB investment only gets you 2TB of storage, but lots and lots of peace of mind.
So, for a small 2 disk NAS, what RAID pattern is correct? RAID1, which makes an exact copy of Disk 0 on Disk1. This way, if one disk fails, you can just yank it and plug in a new one and no data is lost. It does cut your data density in half, but you're not after efficiency, you're after reliability, or so hindsight would inform us.