I agree, it would be expensive to populate all of those slots immediately. I just specced an 8 TB at Newegg for $390 for a NAS/SAN-level drive, you could easily go beyond $1k. But you could also just slot two drives in it and have them mirrored, then as money becomes available add more drives and expand the RAID capability. I paid a lot less than that for a 6 TB bare drive, but it wasn't a SAN drive, which would probably be desirable.
Another thing to consider is sourcing drives. If you were to buy five drives, for example, same make and model from the same dealer, they would probably be from the same manufacturing batch. Which means all of them would have about the same MTBF. The problem here is that when one drive fails, you replace it, and the SAN begins rebuilding it: stressing the other drives that are also probably pushing their max life, and you're likely to have additional drive failures. It's best to mix up the drives to get them from different vendors so you're likely to get different batches and reduce the chance of cascading failures during rebuilds.
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I agree, it would be expensive to populate all of those slots immediately. I just specced an 8 TB at Newegg for $390 for a NAS/SAN-level drive, you could easily go beyond $1k. But you could also just slot two drives in it and have them mirrored, then as money becomes available add more drives and expand the RAID capability. I paid a lot less than that for a 6 TB bare drive, but it wasn't a SAN drive, which would probably be desirable. Another thing to consider is sourcing drives. If you were to buy five drives, for example, same make and model from the same dealer, they would probably be from the same manufacturing batch. Which means all of them would have about the same MTBF. The problem here is that when one drive fails, you replace it, and the SAN begins rebuilding it: stressing the other drives that are also probably pushing their max life, and you're likely to have additional drive failures. It's best to mix up the drives to get them from different vendors so you're likely to get different batches and reduce the chance of cascading failures during rebuilds.