Unraid has three different licenses you can use, Basic (free), Plus ($69), and Pro ($119). Look at the license page for a description on how they differ.
- Runs off of a USB flash drive. You don't lose a SATA port or disk for the operating system.
- Mix and match hard drives and sizes. You can put drives of any size in the array. You only lose the largest disk in the array for parity. (Similar to a Drobo)
- Web interface. A simple web interface lets you configure everything.
- Supports various protocols including SMB, AFP, and NFS.
- No vendor lock-in. Although you're using the proprietary Unraid system, all data is stored on the disks in the ReiserFS filesystem
- Since it's using ReiserFS, if you lose more than one disk at a time you'll be able to get data off the rest of the disks in the broken array.
- Use your own hardware. I built a little server with an Atom processor for less than the price of the cheapest Drobo.
- Set it and forget it. I've had my system running for almost two years
- Stable. I set mine up and have basically forgotten about it. It just works and has been since May 2011 when I configured it.
- Spins down drives when not in use, unlike RAID-5.
- Support and community. There's a very active forum and wiki to help you with any issues, add additional functionality, and much more.
- Price could be. I purchased the $70 license. If you only have 3 disks and don't require some of the extra features, there is a free version.
- Write speeds. If you have the need to push stuff to the server at more than 30MB/sec, this may not be the best choice. If you're just archiving stuff, it really is not an issue. You can also speed the writes up with a cache drive.
Why I Chose UnraidThroughout my history of having redundant storage at home I've had two previous setups that really failed me.
The Drobo is actually a really nice device. The newer ones are supposed to be a lot faster and some even have sharing capabilities. It's just too expensive and they've got you with vendor lock-in.