...thanks, not sure I fully get the Raid 0-5 terminology and functions.
This is good to know for anyone who wants to build a home server or NAS. These questions will arise anyway, either during initial setup, or a short while down the road.
Essentially, RAID can be had in four relevant ways:.
RAID 0 means that you put two identical disks next to each other, and let the computer store every other block of data on each disk. Theoretically, you can almost double the throughput, compared to using one drive, and you get disk space corresponding to both disks (i.e. 2 x 2 TB = 4 TB).
The drawback with RAID 0, is that if one disk would crash (which will eventually happen), your data is gone. You need to restore everything from the backup you hopefully made.
RAID 1 means that you put two identical disks next to each other, and mirror them. It's about as fast (or slow) as writing to a single disk, but if one disk would crash, you still have your data on the mirrored disk. (2 x 2 TB = 2 TB + a full mirror)
In a RAID 5 system, you need at least 3 disks, where data is striped over all but one of your disks, and then parity information is written to the final disk, in a round robin pattern. If one disk would crash, you can restore the information you had on it thanks to the parity data you had on it. It's a relatively cheap way of gaining lots of disk space with some crash resilience. (3 x 2 TB = 4 TB + parity data - but 4 x 2 TB = 6 TB + parity data - you "lose" less capacity the more disks you have in the RAID set).
Drawbacks with RAID 5: Writes are relatively slow, since they basically force the computer to write some data, then read back what it wrote, and finally write the parity information. The more disks you have, the higher the risk that more than one disk will crash before you've had the time to re-sync the RAID set after a malfunction (remember that disk longevity is counted as meantime between failures). In a home setting this is usually not a critical problem, but if you set up more than 8 identical disks in a RAID set, this is a factor to be taken into consideration.
RAID 10 (or 1+0) is a setup that's a bit expensive for home usage, but I want to include it here for two very important reasons: Speed and resilience.
What you get from RAID 10 is a level of performance only matched by RAID 0, along with a robustness only matched by RAID 1. That's because you stripe mirrored disk sets (4x2 TB = 4 TB + a full mirror copy).
The only drawback to RAID 10 is cost. You need twice as many disks as you need disk space.
Summary:
For a home solution, if you can live with a couple of days of downtime when a disk crashes, a RAID 0 protected by regular backups is an acceptable way to go. In such a case, a relatively slow server (but not a low-end NAS) should be able to push data as fast as your network connection can handle.
The luxury version of this is to go for a RAID 10. It doesn't eliminate the need for regular backups, but you're unlikely to have to resort to them even if two disks would fail at once.
If you're only storing data with no particular performance needs on your server (music, films, Time Machine backups, Office documents), then a RAID 5 may be acceptable. Again, keep a good backup routine: If two disks would die for you, your data is gone.
Synology and other NAS makers have proprietary solutions for maximizing the available disk space. This is usually based on the RAID 5 concept, but applied to logical volumes rather than physical disks. In all likelihood it will be slower even than a pure RAID 5, and it won't be more resilient to hardware failures. Also, if the actual NAS box would crash, you're probably in deep trouble. As you understand, I do not recommend going this way unless you really have to - but since you're on a Mac forum, I don't think your financial situation is quite
that bad.
