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

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

RustierOne wrote:

Thanks for the reply. There's lots of useful information in your comment and the links. I can't say I understand some of the other posts in this thread, but I did look at the system report as suggested. For my USB devices I found the following:

  • OWC USB 3.0 750 GB, 7200 rpm - 900 ma available, 0 ma required (even when backing up)
  • Toshiba USB 2.0 750 GB 5400 rpm - 500 ma available, 2 ma required (even when backing up)

I don't understand them showing 0/2 mA required, but as USB 3 can give 900mA and USB 2 500 mA, in your setup you're good to go, no need to worry.

Your USB 2 Toshiba probably requires around 200 - 250 mA.

The OWC HDD is a Seagate Momentus with rated power requirement of 700 ma @ 5V. So even with the requirements for the Mercury On-the-Go drive enclosure, the total should be less than 900 ma. I've begun always connecting the USB 3.0 drive first as suggested in the link to Apple. The Toshiba drive is a self contained HDD with no separate enclosure. So I don't know its power requirements. But with USB 2.0 it should be less

You don't need to worry about the connection order. The comments in the Apple link only apply for an Apple device that is requesting more current than the standard, so more than 900 mA for a USB 3 device, or more than 500 mA for a USB 2 device.

Both your devices are none Apple and fall within their respective USB standards. So you can connect them in any order, plug and un-plug without rebooting (but storage needs un-mounting), and so forth, you shouldn't see any difference or have any problems.

If a non Apple device requires more current than the standard, it will usually come with a second USB cable so it can be powered by two ports, and/or an external adapter.

Technically, it may be possible for suppliers to work with Apple to have their devices request extra current. And technically it may be possible that suppliers provide their own software for PCs and maybe even Macs that achieve the same thing. But for Apple to Apple, it's handled by the protocol extension.


Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow