Ever watched a thread get out of hand - I think this one did
Some clarification for you nuts:
Certain RAID configurations require a minimum number of disks in order to operate.
RAID 0 (striping across disks) can have just one disk in the RAIDset but you gain no advantage from just one drive as the data still goes to just one disk. As soon as you add a second, third or more disks to a RAID 0 raidset, then the controller can send chunks of data to all disks (almost simultaneously) thus improving the rate at which data can be read or written.
RAID 1 (mirroring) requires a minimum of 2 disks (but can have more - triplicated mirrors are not uncommon in some applications). It does not give you and speed improvement but it does give you safety if a disk fails in the raidset. Some sites combine RAID 0 and RAID 1 (commonly called RAID 0+1) to gain both security and speed. As you can imagine it is an expensive option.
RAID 3 (parity disk) and RAID 5 (distributed parity) require a minimum of 3 disks. They are a compromise between RAID 0 and RAID 1. They work by computing and storing a checksum on your data which can be used to reconstruct your data if a disk within the raidset fails.
The 8890 can be configured for RAID 0 (striping) or RAID 1 (mirroring) across only two of its disks. It can support 3 disks (4 if you loose the DVD/CD drive) but the extra disk(s) cannot be included in the raid configuration.
The 8790 also supports RAID but only has room for two disks.
For any application that moves a lot of data around, RAID 0 is a real bonus. You can expect a speed up of up to 1.8x of a single disk.
As to increased risk - yes but less than people think (for a mathematical calculation of this go to my review and click on the RAID links in the index). You are almost at the same risk of losing one disk as that of two disks for all intents and purposes. Also the risk of loosing data due to a disk failure (in what ever config) is much lower than the potential loss due to faultly software, physical damage (ie dropping/liquids/etc), theft or loss. I had a software crash just recently that trashed an important file. Luckily I was able to recover it from backups. Statistically disk failure is the least of your worries for your data.
REGULAR (ie daily) BACKUPS ARE FAR MORE IMPORTANT.