Pick the right external drive capacity.
External drives are bought for years, so the sizing input is linear accumulation rather than a snapshot: current data plus monthly additions across the horizon. Dividing by the fraction you are willing to fill converts that into raw capacity, and rounding up to a real product size gives the drive to actually buy. The months-until-full figure is the sanity check on whether the next size up is worth it.
Data at horizon
Data at horizon = current data + monthly addition x horizon months
Capacity required
Required capacity = data at horizon / (1 - free space fraction)
For bulk archive and backup, a mechanical drive is several times cheaper per gigabyte and adequate. For working files edited directly from the drive, an SSD's speed and shock resistance justify the premium.
Manufacturers count a terabyte as a trillion bytes while operating systems display tebibytes. The capacity is as advertised; only the unit differs, costing about seven percent of the displayed figure.