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

Started Mar 29, 2013 | Discussions thread
Najinsky Veteran Member • Posts: 5,739
Shouldn't be a problem

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.

I often run 2 USB drives, both previously with the MBA and now with a rMBP. Never any issues.

You can use System profiler to see how much current the external drives require and how much is available to them. Make sure both drives are plugged in then:

Apple Menu->About this Mac->More-Info->System Report...

Then click on USB in the left column under hardware, and then on each connected drive in the right column to see what it requires and what's available.

My WD 2TB external drive requires 224mA with 900mA available (standard for USB3 on the rMBP). On the USB2 MBA it was 500mA available (USB2 standard, but for the superdrive, it also shows 600mA as  'Extra Operating Current').


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