I think you need both because the drive gets power off the USB. I have a similar adapter but it supplies power via a plug in adapter rather than the USB connector.
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-Evan
Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63
Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250
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