Often no performance increase with this setup, however disks can be added in 1s with RAID 5. One disk becomes a parity disk for the other 3. 4, 2TB drives results in 4TB of usable space Combines redundancy and speed, with half of the drive space usable - i.e. Creates a RAID 0 array from 2 disks, then creates parity for those 2 disks on another 2 disks. Disks can be added in 1s, although performance increase decrease the more disks that are added (diminishing returns). RAID 0: 2 Disks are striped, halving the data onto each disk. If 2 2TB drives are in a RAID 1 array, only 2TB of hard drive space will be available RAID 1: 1 Disk is used for storage, the other used for parity. Again, RAID 0 if redundancy isn't important.

6 Drives - RAID 6 for 2 drive redundancy, if uptime is a huge issue for you, or just plain old RAID 10 if uptime doesn't matter a huge deal.

4 Drives (and up to 6) - RAID 10 for redundancy and extra speed.

I'd be going with RAID 1 for redundancy here.

2 Drives - your only options are really RAID 1 and 0.

Overall, RAID 10 gives you redundancy and extra speed, but only 1 drive can fail in RAID 10 before your data is at risk. That is, it combines the speed you get from RAID 0 by striping two HDDs, but then gives you the redundancy of RAID 1 by making a span of that RAID 0. If you were to start with 4 drives, I'd go with a RAID 10 (and even stay with RAID 10 when you get your 6 drives). Expandability really depends on what hardware/software you're using in most cases, and so you'll have to look up on your hardware's manual on how to expand the RAID volumes. RAID 6's are fairly easy to expand into, but you'd already have 6 drives in there. If you have a hardware RAID controller, performance won't be affected much (assuming that the RAID controller is up to the task and how much throughput you're hitting the RAID config with) and with RAID 6 two drives can fail before your data becomes at risk.

However, if you were to go straight in with 6 drives, I would be putting in a RAID 6.

If you're starting with 2 drives your only two choices are RAID 1 and 0 as these modes only require a minimum of 2 drives, with RAID 10 requiring 4 and RAID 5/6 requiring 4 as well.

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