If its got a SATA power connector on the drive, and the external USB enclosure has a SATA power lead, then (unless the USB external enclosure it out of spec...) there is no chance that the drive will be damaged when you connect it. SATA power is well-standardized, and the only variation is whether the particular SATA power connector happens to supply 3.3V or not.
Old-style Molex connectors supply 5V and 12V, and SATA power connectors supply 5V, 12V, and (assuming they aren't a Molex-to-SATA power adapter) 3.3V. The Wikipedia article says that drive manufactures avoid requiring the 3.3V power source, since in some cases it isn't present.
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