Any Problem Using Two External USB-Powered HDDs on rMBP?

Started Mar 29, 2013 | Discussions thread
Najinsky
Veteran MemberPosts: 4,598
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Re: Shouldn't be a problem
In reply to Doug R, Apr 2, 2013

Doug R wrote:

Najinsky wrote:

Recent Macs without built-in DVD drives; which includes the MacBook Airs, Minis, new iMacs and the retina MBPs, have extra power on the USB bus to support the the external superdrive which connects via USB but draws a bit more power.

They give more power because the specs call for it not to power a specific Apple accessory.

So you're saying this (http://en.wikipedia.org/wiki/Universal_Serial_Bus) is wrong?:

"A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0. There are two types of devices: low-power and high-power. A low-power device draws at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws, at most, the maximum number of unit loads the standard permits. Every device functions initially as low-power—but may request high-power, and gets it if it's available on the providing bus.

Some devices, such as high-speed external disk drives, require more than 500 mA of current and therefore may have power issues if powered from just one USB 2.0 port: erratic function, failure to function, or overloading/damaging the port. Such devices may come with an external power source or a Y-shaped cable that has two USB connectors (one for power+data, the other for power only) to plug into a computer. With such a cable, a device can draw power from two USB ports simultaneously."

And you're saying this (http://support.apple.com/kb/ht4049) is wrong?:

"

Apple peripheral devices may request more than 500 mA (Milliamps) at 5 V (Volts) from a port to function or to allow for faster charging. Such Apple peripheral devices include:

  • Apple MacBook Air SuperDrive (when connected to supported computers)
  • Aluminum Wired Keyboard*
  • iPod
  • iPhone
  • iPad

To meet requests for additional power from these Apple peripherals and devices, some Apple computers and displays can provide up to 1100 mA at 5 V through the port to which the Apple peripheral or device is connected. This power is available under certain conditions:

  1. An Apple peripheral device must be plugged directly into an Apple computer or display. Apple peripheral devices connected to hubs will not have access to extra power above the standard USB specification of the port the device is connecting to (500 mA for USB 1.1 or 2, 900 mA for USB 3).
  2. Your Apple computer or display must be powered on and must be awake. If your Apple computer or display is asleep, all ports will provide their normal maximum output. If your Apple computer or display is powered off, no power will be provided.
  3. The port providing extra power is determined by the first Apple peripheral or device to connect to the Apple computer or display that requires power beyond 500 mA for USB 1.1 or 2, or 900 mA for USB 3. The remaining ports will continue to offer their normal maximum output. Some Apple computer and displays may offer the ability to operate more than one USB port at 1100 mA at 5 V. On those computers, the second or third port is enabled when an appropriate device is connected."

-Najinsky

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