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)