Why one drive has only USB for power and another has USB and power cable?

Discussion in 'Mac Accessories' started by PicnicTutorials, Feb 18, 2014.

  1. PicnicTutorials macrumors 6502a

    PicnicTutorials

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    #1
    External drive that is. Is there a difference here? Is USB only limited in speed or size? Or any other factor?
     
  2. paulrbeers macrumors 68040

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    #2
    USB Power only are generally 2.5" laptop hard drives (and SSDs) because they use a LOT less power than...

    USB + Power brick is generally 3.5" desktop drives since they use more power than the above.

    Generally the 3.5" desktop drive will perform faster than the 2.5" drive, but will you really notice? Probably not....
     
  3. PicnicTutorials thread starter macrumors 6502a

    PicnicTutorials

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    #3
    Thanks! When you say 2.5" are you referring to TB? Or inches?
     
  4. DeltaMac macrumors 604

    DeltaMac

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    #4
    Just a choice in design for a drive enclosure.
    Portable drives (usually have 2.5-inch devices) may be USB bus power-only, or may have an optional external power.
    Larger drives (with 3.5-inch devices) will not be bus powered.

    I usually shop for portable drives that can take external power, too.
    I often will plug in an external power source when doing a large transfer. It makes me imagine that the copy or install process will be more stable.

    Not sure what you mean by your question
    USB has no size limit, other than whatever device is using that bus.
    USB speed is limited by the bus that is available (plug a USB 3-capable device into a USB 1 bus will limit the device to only USB 1 speeds)

    The little (") quote symbol refers to inches. What do you mean by TB (which has nothing to do with USB?)
     
  5. Tomorrow macrumors 604

    Tomorrow

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    #5
    The " after the number represents inches. Terabytes would be Tb.
     
  6. PicnicTutorials thread starter macrumors 6502a

    PicnicTutorials

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    #6
    Yes I know what each mean. So he is saying USB only is generally 2.5 inches wide/long? Are you talking about the guts? What.

    ----------

    USB powered only. Aka no power brick.
     
  7. DeltaMac macrumors 604

    DeltaMac

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    #7
    I have never seen a full-size USB enclosure (with 3.5 drive) that is bus powered. All are self-powered (either a brick, or internal power supply in the enclosure)
    Portable USB storage (2.5-inch drive) may be either bus or self powered, or just bus-powered - at the whim of the manufacturer.
     
  8. PicnicTutorials thread starter macrumors 6502a

    PicnicTutorials

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    #8
    Thanks for the answers but I do don't know why there is confusion.

    one drive has only a USB input. Another has USB and a power cord input. Question in another way... Can a USB only have a 3TB storage capacity? Or is it limited to less? When you say 2.5" I still have no idea what your referring to? The outer enclosed size? The spinning disk size? The width. The height?
     
  9. paulrbeers macrumors 68040

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    #9

    The 2.5 inches is the size of hard drive.... the physical size of the hard drive....

    http://en.wikipedia.org/wiki/Hard_drive#Form_factors

    You can't have a 3.5" hard drive powered by USB bus only...

    Edit: And no you can't have a 3TB USB Bus powered only drive since the largest 2.5" drives are current 2TB. 3TB only exist in the 3.5" drive...
     
  10. DeltaMac macrumors 604

    DeltaMac

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    #10
    2.5-inch is the actual device inside the enclosure.

    Most makers will refer to that as a portable storage.
    I think most portable drives come as bus-powered, and an external brick may be optional. Some DON'T have the external brick as an option, so will be USB bus power only.

    You did ask "why?" - and the answer is: Ask the manufacturer of the enclosure. It's purely a marketing choice.

    You COULD have 3 TB or more with 2.5 drives (multiple) in a single enclosure - but that would not be bus-powered, because of the power required for multiple drives inside the enclosure.
     
  11. PicnicTutorials thread starter macrumors 6502a

    PicnicTutorials

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    #11
    Cool! Ok got it. So the drive size dictates the size of the storage. And USB only does not go beyond 2.5". So USB only is limited to around 2TB or less drives. I wonder if rpm has anything to do with it too. I ask because I have a backup of my backup that I keep unplugged. But it has both power and USB and therefore turns into a cord mess each time I pull it out to plug it in. Vs my 500G one that just has a 12" USB cord to contend with is so much nicer.
     
  12. thetman macrumors member

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    Dec 30, 2003
    #12
    Some 2.5" hard drives are available as 7200rpm vs the more common 5400rpm. Those most likely have an extra power requirement and that MAY be enough to require a second USB port for additional power.
     

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