6 Replies Latest reply: Mar 17, 2012 12:54 AM by Courcoul
temhawk Level 1 (35 points)



I am going to buy a new 17" MacBook Pro soon and I've been doing research on how to customize it when I get it. I want to install my own SLC SSD as the internal drive because it's cheaper and has a longer life expectancy. Because of the limited capacity of SSDs these days, I am also going to be getting an external HDD to keep all my data that would not benefit greatly from SSD speeds, as well as for backups of everything on the internal SSD.


I wanted this external HDD to meet three criteria:

  • 1TB capacity
  • 7200rpm speed
  • 2.5" size, so that I can use it without needing to plug it into a power socket


There aren't many options to choose from that meet those requirements. In fact, I only found one candidate, the “Seagate Constellation.2”. It looks like a good set of specs and Seagate is a reputable hard disk maker I've read, so I'm just about ready to buy one. My only point of concern is whether it can be powered and operated solely over FireWire, or if I would have to use a power adapter.


So my question is, can the 17" MBP (Late 2011) supply enough watts/volts/amps over FireWire 800 in order to power this Seagate disk? (Sorry for rambling too much)


I have looked at the spec sheet and read up on IEEE 1394, i.e. FireWire, and I wasn't able to come up with a definitive answer, and I'm not even confident about how these electronics operate (specifically if the watt/volt/ampere requirements have to be met precisely or if they can go over or under a little, or a lot). I hope someone helps me out here.


Now here are the relevant specs, from the ST91000640NS product page:


12V start max current0.85A
5V start max current0.51A
Average idle power2.95W
Average operating power5.43W


According to Wikipedia's article on IEEE 1394, FireWire can supply a maximum of 45W and 30V.


Apple's FireWire Ports Specifications article says:


Input voltage range8 to 33 V
Input power0.3 W
Output voltage range12 V to 30 V (varies by product)
Output powerminimum of 7 W per port


The disk will be put in an OWC Mercury Elite Pro Mini enclosure, by the way. (Though I don't think that matters.)

MacBook Pro, Mac OS X (10.7.3)
  • Kappy Level 10 (266,041 points)

    In a word, "Yes."

  • temhawk Level 1 (35 points)



    I rambled too much, heh.

  • eww Level 9 (52,980 points)

    Kappy may be right — he most often is. But the Constellation is an enterprise drive designed for continuous use in servers, not for bus-powered use as an external drive. Among currently-available 1TB notebook drives, the kind that are normally used in bus-powered portable external enclosures, there are none that run at 7200 RPM, so I assume that's why you've settled on the Constellation. My concern about your plan is twofold: first, the drive is 15mm high. Will it fit in the enclosure you've selected? Ask OWC. And second, because the Constellation is normally run on AC power without any amperage ceiling that it's likely to exceed, I wonder whether during spinup or heavy usage it will occasionally draw more power than your FireWire bus can supply, causing it to be automatically powered off. I'm not conversant enough with electrical specs to be able to tell whether that's a possibility. Perhaps Kappy is.

  • Kappy Level 10 (266,041 points)

    Well, I wasn't trying to make a hint, but since you asked: yeah, a little. Was really a short question. I wasn't trying to be curt, just informative.

  • temhawk Level 1 (35 points)

    Thank you.


    I checked that the enclosure fits the disk – it does. I will contact Seagate to ask about spinup requirements.

  • Courcoul Level 6 (12,996 points)

    If I read the specs correctly, FW has a max of 1.5A available on the 30v power pin, in comparison to USB2's 500mA. However, trying to draw that kind of power from the battery will seriously drain it in short order AND given that the MBP's 85W power brick has a DC current rating of max 4.6A, you'll probably be able to warm your breakfast muffins on top of it.


    Another FW detail. If you want max performance, data-wise, use the shortest cable possible.