RAID5 is indeed too dangerous to use, for the reason Tom has explained. That's why a lot of 'hardcore' home users started using RAID6, where two drives may fail. This is course doesn't help with the slow rebuild times and scrubbing etc... It also means you'll need at least 4 drives and a lot of drive space simply for parity calculations so you won't have that much free space left. (if you only use 4 disks, you'll loose half of your disk capacity, which makes it rather costly)

If I was to use a 'regular' RAID system, I'd use ZFS (which isn't 'regular RAID' in any sense, but you probably get my drift). ZFS allows for up to triple parity, but here also double is more than sufficient. The big plus about ZFS is that it's self-healing, is a copy-on-write filesystem and because of this can use snapshots which are very handy.

The only reason I didn't start using ZFS is because, as I understand it, it's not possible to add disk space to a created RAIDZ array in a drive pool. It is possible though to add disks to the drive pool and to create additional RAIDZ arrays, but that's not exactly what I want. Like most users, I simply want to see one big volume, containing all my disk space. The big plus of ZFS is it's robustness and it's speed.

But of course there is a downside to ZFS as well, it being the demands of ZFS are pretty high if you want to use it to the best of it's capabilities, especially the memory demands (and this is especially true if you want to use de-duplication). On the other hand, it allows you to use an SSD cache drive which speeds things up considerably. But on the other side, I use a gigabit network at home. The theoretical maximum speeds is 125 MB/s (if I don't use any link aggregation, which I could do but don't). So if the server it capable of providing that speed, it's sufficient, because I don't think I'll benefit much from having SSD-type speeds available if the network can't keep up with it.

So all this has led me back to unRAID. But now I start reading stories again of 50 hours of 'preclearing' drives (making new drives ready for use with unRAID - a mandatory part of the procedure) before you can start to use it, or days to calculate the first parity calculation (so it takes days before you are somewhat protected). Also, the speed of unRAID isn't all that good. I read about speeds of 40-50 MB/s which I find not very impressive.

In other words - I'm still not quite sure which route I'll take. My demands are rather simple though: I want a NAS system which will provide me with speedy file transfers (+100 MB/s), drive pooling (with easy expansion when full) and some form of protection (don't worry, I'll still keep a separate backup as well). As far as I can tell, there is no do-it-yourself software available yet which gives me all those options. Commercial NAS'es like Synology also are not an option, both because of their price and because they still use old RAID technology and no drive pooling.