Capacity Planning Starts With What You Already Hold
The enclosure is the cheap part of network storage. Drives account for most of the cost, all of the capacity and nearly all of the risk, and they are the part of this decision that has changed most over the past year. Sizing an array well means measuring what you store today, estimating honestly how quickly that grows, and buying for the point where the array is comfortably full rather than the point where it is technically full.
Two figures then decide the drive itself. The first is the workload rating in terabytes written per year, which is what separates a desktop disk from one specified for a multi-bay enclosure that never stops spinning. The second is capacity per drive, because that sets how long the array takes to rebuild after a failure and therefore how long it spends in its most exposed state.
