Having trouble deciding

zarra - I'm not sure you are really appreciating the advantages of X-Raid2. You first decide the approximate storage space you need - and for now assume 6TB is what you need, not what you want. You can achieve that volume with 4x2TB or 3x3TB drives, and I also recommend the WD Reds. I have 2x1TB WD10EFRX drives (one came with a new NAS and the other was purchased on one of Newegg's weekend sales. The nominal price of the WD20EFRX at Newegg is $160 (with shipping) and the nominal price of the WD30EFRX is $217 (w/ shipping). So the 4x2TB array would cost $640 and the 3x3TB array would cost $651 yet leaves an empty slot for future expansion. (Note: the actual disk space in both examples after overhead and formatting is approximately 5.4TB). X-Raid2 in a four bay environment is essentially expandable Raid5 which uses one drive for redundancy. (With only two drives it is Raid1, still with one drive for redundancy). Your proposed solution of two Raid1 volumes with 4x3TB drives would cost an extra $217 for the same 5.4TB (net) storage space.

Do not mistake the redundancy for a viable backup. A viable backup requires a separate device, be it a separate external drive (or two), another NAS (which is the fastest and easiest but most expensive) or cloud backup. All hard drives can, do and will fail. It is not a question of if, but when. I once had a drive fail suddenly overnight within a month after purchase, yet it twin (part of a two drive array) ran for over three years in 24/7 RAID service winding up at the end as part of a four drive array in the same NAS it started in. It was retired to occasional use and is still serviceable today.