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I wonder in what cases a hard drive enclosure has a separate power supply port different from its USB port, and in what cases its power port and USB port are the same?

Is the size a determining factor? For example, do 3.5" enclosures and bigger all have a separate power supply port, and those smaller (such as 2.5") all have a shared port for power and USB?

What else can be determining factors?

Thanks!

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closed as not constructive by Ƭᴇcʜιᴇ007, 8088, Tom Wijsman, Diogo, Joe Taylor Aug 13 '12 at 12:26

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It depends on the power draw of the specific drive, and the design chosen by the manufacturer. –  Ƭᴇcʜιᴇ007 Aug 10 '12 at 17:47

2 Answers 2

up vote 7 down vote accepted

The only thing that matters is how much power the drive consumes. In general, large 3.5" hard drives draw more power then what even two USB ports can provide, whereas smaller 2.5" hard drives can be powered from one or two USB ports.

For reference, a single USB port can provide a maximum 500 mA of current. A 500GB Western Digital Caviar Blue 3.5" hard drive, on the other hand, requires 1220 mA at idle, and 1360 mA when reading/writing files to/from the drive. The 2.5" version of the drive (500GB WD Scorpio Blue) requires significantly less power, drawing 400mA at idle and 500 mA when reading/writing.

It is part of the USB specification that if a device attempts to draw too much power, it is simply disconnected from the target system. This is why some enclosures require you to plug in two USB cables. However, to get a full 3.5" HDD working, you'd need the power of almost four USB ports to get the HDD working, which is also why you never see full 3.5" drive enclosures without an external power supply.

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Generally, 2.5" drives can be USB powered. 3.5" drives often require more power than USB specifies.

USB 2 allows max 500 mA at 5V nominal.

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