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You are here: MacNN Forums > Hardware - Troubleshooting and Discussion > Consumer Hardware & Components > 2.5" hard drives, enclosures, and bus power

2.5" hard drives, enclosures, and bus power
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slugslugslug
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Jul 5, 2010, 10:39 AM
 
If I want to take a laptop hard drive and put it in a USB enclosure or drive, is there any way of predicting whether it can get power from just one USB port? A few years ago, when I swapped my MacBook’s drive into my PS3 and the PS3 one into an enclosure, I was disappointed to find that I had to use one normal A-to-mini-B cable and one USB to random-round-AC-port cable. I was never really sure if this was the fault of the drive’s power consumption or the enclosure (its manual did seem to imply that the 2nd cable might not always be necessary).

So, is the need for 2 USB ports strictly a function of the drive? If so, is there something to look for on the standard spec list on NewEgg, or will I have to do a bunch of googling for any drive I consider getting? Or is it some magical combination of drive and enclosure?

Also, I realize that the single-cable problem could be solved by using my MacBook’s lonely FireWire port, but if I have a portable hard drive, I might want to drag it around to my PS3 or work Dell, so I’d still want USB for flexibility. Thanks in advance.
     
ghporter
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Jul 5, 2010, 11:59 AM
 
Current laptop drives are usually just fine with a single USB connection. The commercially available portable hard drives all use a single connection.

Read the specs on a drive you're interested in to see how much its maximum current draw is-this draw will be when the drive spins up from stopped. For example WD's Scorpio Blue 1TB laptop drive is rated at 500mA for read-write operations, and at .985A max draw. A standard USB 2.x port should easily handle that.

Unfortunately one often has to pilfer through a drive maker's site to find the detailed specs, but it's worth it most of the time.

Glenn -----OTR/L, MOT, Tx
     
mduell
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Jul 5, 2010, 05:42 PM
 
Most 2.5" hard drives are right around the limit for USB power, so a lot of manufacturers throw in the dual ended cable just in case even though it's not always needed. Check the spec sheet for your 2.5" drive to see if you're over the USB current limit.

Originally Posted by ghporter View Post
Read the specs on a drive you're interested in to see how much its maximum current draw is-this draw will be when the drive spins up from stopped. For example WD's Scorpio Blue 1TB laptop drive is rated at 500mA for read-write operations, and at .985A max draw. A standard USB 2.x port should easily handle that.
USB 1.x/2.x power is limited to five 100mA units. How are you going to pull an amp??
Even 3.x only raises it to six 150mA units.
     
slugslugslug  (op)
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Jul 5, 2010, 07:00 PM
 
So it sounds like you’re both saying that how many cables I need should depend entirely on the drive and not the enclosure. That’s good to hear.

On the other hand, I’m still in the dark about exactly what specs I should be looking at. From Mark’s post and a cursory glance at Wikipedia, it seems like I shouldn’t ever expect to use anything over 500 mA ever. But after reading Glenn’s post, I found some specs and saw that a bunch of drives list a max of ~1 A. Which would make it seem like, contra both of y’all, most drives are over the standard limit.

Now, given that I’ve seen external drives in enclosures with only a single USB port, it’s clear that that’s not the case. So I guess the “peak" part isn’t what I should look for, so my question remains. What’s the specific spec and its limit? Read/write and 500 mA or something else?

Thanks again.
     
ghporter
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Jul 5, 2010, 08:19 PM
 
I was under the impression that a single device on a USB bus could pull as much as 1A by itself if it was properly set up... Doesn't an iPhone require 1A for full charging?

Glenn -----OTR/L, MOT, Tx
     
mduell
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Jul 6, 2010, 12:32 PM
 
Originally Posted by ghporter View Post
I was under the impression that a single device on a USB bus could pull as much as 1A by itself if it was properly set up... Doesn't an iPhone require 1A for full charging?
Apple has done a variety of non-standard things with USB power, including their latest proprietary "high power" USB (2A) for the iPad.
     
ghporter
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Jul 6, 2010, 08:27 PM
 
I've charged my iPhone from "standard" PC USB ports too... Not "slow as molasses" charges either...

Glenn -----OTR/L, MOT, Tx
     
mduell
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Jul 6, 2010, 09:25 PM
 
500 mA is about 2 hours for the 80% of the iPhone battery you can charge quickly. Is the slow as molasses version when it's only pulling one 100mA unit? That'd be 12 hours for a full charge.
     
ghporter
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Jul 6, 2010, 09:37 PM
 
The point here was that commercially available portable drives often use notebook drives, and they work from a single USB port. For example, the WD My Passport runs on a single USB 2.0 port. Unfortunately I can't find detailed specs, such as current draw. I probably overstepped when I used the Scorpio Blue as an example, but these little boxes must have notebook drives in them so it was logical that some notebook drive or other would work on a single USB 2.0-spec port.

Glenn -----OTR/L, MOT, Tx
     
   
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