Plan how much drive space you need.
Sizing a drive for today guarantees a shortage within two years. Projecting current data forward at your growth rate, then dividing by the fraction you want left occupied, gives the real requirement — keeping twenty percent free means dividing by 0.8, not subtracting twenty percent. Free space matters technically too: SSDs need spare blocks for wear levelling and slow down markedly when almost full.
Projected data
Data in three years = current data x (1 + annual growth) ^ 3
Capacity with headroom
Capacity needed = projected data / (1 - free space target)
Ten to twenty percent is the usual guidance. Write performance degrades as the controller runs short of free blocks to write into, and the effect becomes obvious above ninety percent full.
Two drives let you separate a fast system drive from bulk storage and reduce the loss if one fails, but a single drive is simpler and avoids splitting libraries. Neither replaces a backup.