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

PicnicTutorials

macrumors 6502a
Original poster
Dec 29, 2013
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External drive that is. Is there a difference here? Is USB only limited in speed or size? Or any other factor?
 

paulrbeers

macrumors 68040
Dec 17, 2009
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External drive that is. Is there a difference here? Is USB only limited in speed or size? Or any other factor?
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....
 

PicnicTutorials

macrumors 6502a
Original poster
Dec 29, 2013
544
12
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....
Thanks! When you say 2.5" are you referring to TB? Or inches?
 

DeltaMac

macrumors G3
Jul 30, 2003
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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
Is USB only limited in speed or size?
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)

When you say 2.5" are you referring to TB? Or inches?
The little (") quote symbol refers to inches. What do you mean by TB (which has nothing to do with USB?)
 

PicnicTutorials

macrumors 6502a
Original poster
Dec 29, 2013
544
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The " after the number represents inches. Terabytes would be Tb.
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.

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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?)
USB powered only. Aka no power brick.
 

DeltaMac

macrumors G3
Jul 30, 2003
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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.
 

PicnicTutorials

macrumors 6502a
Original poster
Dec 29, 2013
544
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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?
 

paulrbeers

macrumors 68040
Dec 17, 2009
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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.

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USB powered only. Aka no power brick.

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...
 

DeltaMac

macrumors G3
Jul 30, 2003
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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.
 

PicnicTutorials

macrumors 6502a
Original poster
Dec 29, 2013
544
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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...
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.
 

thetman

macrumors member
Dec 30, 2003
45
2
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.
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.