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

Started Mar 29, 2013 | Discussions thread
gaussian blur
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Re: Shouldn't be a problem
In reply to Najinsky, Apr 3, 2013

Najinsky wrote:

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.

Apple states in their tech note (which I linked) that the MacBook Air 2008 only sources 500 mA.

Since the external SuperDrive worked on that machine, it only needs 500 mA. It may run a little better with 600 mA (which isn't really all that much more), but 500 mA is all that's necessary. With the plist patch, the drive will work on other Macs that only source 500 mA.

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