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. However, if you were to go straight in with 6 drives, I would be putting in a RAID 6. 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. RAID 6's are fairly easy to expand into, but you'd already have 6 drives in there. 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. 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). RAID 10 combines RAID 1 and 0. 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. Overall, RAID 10 gives you redundancy and extra speed, but only 1 drive can fail in RAID 10 before your data is at risk.
SO, an overview of your choices:
- 2 Drives - your only options are really RAID 1 and 0. I'd be going with RAID 1 for redundancy here. RAID 0 if redundancy isn't important.
- 4 Drives (and up to 6) - RAID 10 for redundancy and extra speed. 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. Again, RAID 0 if redundancy isn't important.
UPDATE
An overview of RAID:
RAID 1: 1 Disk is used for storage, the other used for parity. Disks can only be added in 2s in RAID 1. If 2 2TB drives are in a RAID 1 array, only 2TB of hard drive space will be available
RAID 0: 2 Disks are striped, halving the data onto each disk. Disks can be added in 1s, although performance increase decrease the more disks that are added (diminishing returns). All data from hard drives is usable.
RAID 10: 4 Disks required, minimum. Creates a RAID 0 array from 2 disks, then creates parity for those 2 disks on another 2 disks. Drives must be added in pairs. Combines redundancy and speed, with half of the drive space usable - i.e. 4, 2TB drives results in 4TB of usable space
RAID 5: 4 Disks required, minimum. One disk becomes a parity disk for the other 3. Often no performance increase with this setup, however disks can be added in 1s with RAID 5. Drive usable space is 3/4 of total drive space, however only 1 disk stays as parity i.e. 4, 2TB drives results in 6TB usable space; 5, 2TB drives results in 8TB usable space. RAID 5 does have a drive limit, however it is not worth mentioning as your setup will not reach this limit.
RAID 6: 4 Disks required, minimum. Two disks become parity, with the other 2 used for volume space. Drives can be added in 1s. 2 Disks can fail in this setup before data corruption/loss starts to become an issue. With 4 drives, half of drive space is usable, however only two disks are parity regardless of total volume disks. i.e. 4, 2TB drives gives 4TB usable space; 5, 2TB drives gives 6TB usable space.
And a final note: RAID configurations are NOT a substitute for backups.
