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 gaussian blur, Apr 3, 2013

gaussian blur 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.

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)

That part is wrong. The external SuperDrive only works on supported computers, which are Macs that lack an internal DVD drive, unless you patch a plist or replace a custom bridge chip. It has nothing to do with power requirements.

No it's still right, it draws 600mA (shown as Extra Operating Current), 100mA above specification. The reason the hack works is because even Macs with an internal superdrive now provide extra operating current for charging iPhone and iPads.

The external SuperDrive came out with the original MacBook Air in 2008. According to Apple's own documentation, that MacBook Air only sources 500 ma. That page at apple.com has been removed, but it's still in the Wayback Machine.

Another person tried to figure out why the external SuperDrive only worked on the MacBook Air. She tried various things, including adding additional current on both Mac and PC. It didn't help. What did work was replacing the custom bridge chip, which apparently is part of a secret handshake, with a standard bridge chip. A bit of soldering skill required, but it fixed the problem.

Later, someone figured out where the Mac checks for the drive, and by patching a plist file, the check always passes and the drive always works, regardless of which Mac, including ones that cannot source more than 500 mA.

The superdrive requires 600 mA to operate reliably; presumably writing at maximum speed will be the biggest draw where this is needed. It's not uncommon for drives and devices to still operate with reduced power, but the risk is that it's operating with reduced performance/reliability.

The protocol at the time required devices to start in low power mode, 100mA and then request the additional power. As per the standard, as a USB 2 device it can request 500 mA and receives 500 mA. This is what gets shown on the system report.

The 600 mA comes by way of a protocol extension and this gets shown on the system report as Extra Operating Current. In the absence of reliable documentation I think the only way to know for sure what the MBA 2008 does is to check the system report. My money's on the 2008 MBA supplying the 600 mA EOC and the documentation being incomplete.

On the HT doc I linked to is says:

"Note: Apple computers and displays that were introduced before 2007 support only 500 mA at 5 V from their ports and do not offer additional power."

As the MBA 2008 is from 2008, I think it's a good bet it supplies the EOC.

On older Macs with the hack, I guess there's a chance it's getting by on 500 mA, perhaps accompanied with a higher occurrence of write errors at full speed.

I guess we're both a little bored today?

-Najinsky

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