What fraction of external hard disks is still "shuckable" these days? I know the WD Passports aren't. The USB connector is directly on the drive's PCB, no SATA bridge.
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It's easier to cut the 3.3v wire from the power supply. If you have color coded wires, it'll be the orange wire; if not, it'll be the same color as every other wire.
To a first approximation, nothing ever uses 3.3v from sata power, so it made sense to remove it from the spec. Reusing the pins so that 3.3v inhibits functionality was kind of crazy though.