Can I use two video cards on two different boot drives with two different OS?

Discussion in 'Mac Pro' started by Slayerprime, Jul 18, 2015.

  1. Slayerprime macrumors newbie

    Jul 18, 2015
    Greetings Mac Rumor Forums!

    My question is about using two video cards: One for a bootdrive running Mavericks, and a second card for for a second bootdrive running 10.6.8 Snow Leopard.

    I have recently updated to the Sapphire Radeon 7950 with 3 gigs of ram from a Radeon 5870 with 1 gig.

    The new Sapphire works fine with my main HD with Mavericks, however, as you may know, Sapphire is not supported with any OS before 10.7. My second hardrive, which I use for legacy apps and games is a 10.6.8 Snow Leopard drive. While I can boot into 10.6.8 with the Sapphire card, you basically cannot do anything except move files around - no games run, and apps like Photoshop run like it's 1995 again.

    I still have room for a second card, so my question is: can I re-install my Radeon 5870 as a "second card" and would my 10.6.8 Snow Leopard HD then be able to function again?


    P.S. I have seen minimal gains so far moving to the Sapphire card. Video games on full settings only run about 5-10 frames per second faster, and no real gains in apps like Photoshop to speak of. So if the answr to my question above is "no", I may just return this Sapphire card and go back to my previous card.

    MacPro 4,1 | 32 gigs ram | 10.6.8 | 10.10.4
  2. h9826790, Jul 18, 2015
    Last edited: Jul 18, 2015

    h9826790 macrumors G4


    Apr 3, 2014
    Hong Kong
    The quick answer is no, due to no extra power avail for the 5870.

    The real answer is yes, but I will say no point to do so (in your case). Especially when considering the 5870 driver optimisation is really good in OSX.

    Unless you gaming at high resolution (e.g. 1440P with ultra texture). There is not much chance that 3G VRAM will show any difference in gaming.

    However, 5-10 FPS is not negligible in some case. And that's actually quite an improvement. Unless you are talking something well above 60FPS.

    For me, the 7950 is mainly for 4k video editing in FCPX. For photoshop, even my stock 4870 is more than enough in general.
  3. Synchro3 macrumors 68000


    Jan 12, 2014
    Connect the display to the Radeon 5870 before booting OS X 10.6.8, or connect the display to the Radeon 7950 before booting OS X 10.9. AFAIK the card without a display connected will be ignored and drivers will not be loaded.

    But you have to organize power for both cards.
  4. Slayerprime, Jul 18, 2015
    Last edited: Jul 18, 2015

    Slayerprime thread starter macrumors newbie

    Jul 18, 2015

    That is slightly annoying, but not that horrible. I think I may try that. Can I use splitters on the two power lines that I am currently using for the Sapphire? My internal power supply should be able to handle the two cards, right?
  5. Slayerprime thread starter macrumors newbie

    Jul 18, 2015

    Running World of Warcraft with maxed graphics, on the original 30 inch Apple Cinema Display. I was getting 20-ish frames per second in the 'home base' garrison. With the Sapphire I am now getting 25-ish frames. It does make a difference with being able to move around smoother - but it was not as big of a gain as I received when I upgraded to the 5870 from the stock card obviously. However, you are correct that when raiding with a team of 30 people in WoW, a gain of 5 frames per second can be quite valuable to player performance.
  6. Synchro3 macrumors 68000


    Jan 12, 2014
    There is no big difference between a Radeon 5870 and a Radeon 7950 regarding games. You should have made the investigation before, it's known. For example with a GTX 680 you get around 120 fps in WOW.
  7. Slayerprime thread starter macrumors newbie

    Jul 18, 2015
    I found this answer about supplying power to both cards:

    "I have multiple video cards in all of my Apple Mac Pro's.

    The Mac Pro mainboard has two six-pin power ports (75W ea). You use a Mac Pro PCIe 6-pin power cable such as this one here: -GeForce-8800-GTX-285-/160708022317

    If you already have a 5870 installed in your Mac Pro, then you probably already have two of the cables listed above already installed. (with one cable going to each of the two 6-pin power connectors on the 5870 graphics card that you currently have installed).

    So for the first video card, you are drawing 150W (75W x 2) directly off of the mainboard, and then the card is drawing an additional 75W off the PCIe bus (the card slot), for a total of 225W (maximum).

    For the second video card, you are saying that you don't have any additional "6-pin" power cables to connect to. Yes, this is correct. What you do is simply get a 4-pin MOLEX to PCIe 6-pin (or 8-pin, which is 6+2 pin) power cable adapter.

    This will simply draw the power off of your 12V power supply leg and convert it so you can use it to power a second video card.

    You use this cable here for your second video card: 8A6Q

    It's about $6 for the cable, and it converts two (dual) 4-pin Molex (75W x 2) into dual 8 (6 + 2) pin PCIe power. It has two connectors that can be used as either two 6-pin or two 8-pin connectors (6+2). Some graphics cards (such as the Radeon HD 7970 require an 8-pin power connector, and a 6-pin, while other graphics cards such as the HD 5xxx series only require two 6-pin power connectors).

    The adapter that I listed (for $6 at Fry's) will do exactly what you need it to do. It will draw 150W of power (two 4-pin Molex) and convert it into two six-pin (or or one 6-pin and one 8-pin, or two 8-pin) connectors and you just plug those two six-pin connectors into your video card, and it will just draw the power from your 12V power supply leg (instead of drawing power off of your motherboard). It's better to go directly through the power supply anyways.

    The HD 5870 that you are talking about will specifically use two 6-pin connectors, so you just use the $6 cable adapter that I listed (from Fry's) and just use the two 6-pin connectors and plug them directly into your new second HD 5870 graphics card.

    If you were trying to get into the larger (and more "power hungry") video cards like the GTX 680 or GTX 690, or Radeon HD 7970 or HD 7990. Those require one six-pin, and one eight-pin, so you would simply just use the same exact Fry's adapter, and draw the 8-pin power from the two 4-pin Molex (75W x 2 = 150W), then use a six-pin from your motherboard for the other 75W (and draw 75W off your motherboard). If you want to install two Radeon HD 7970 graphics cards, then just buy two of these Dual 4-pin Molex to 8-pin (6+2) PCIe cables, and use one 8-pin for each graphics card, and then use one six-pin from each of two 6-pin ports on the motherboard. That way you are "splitting" up the power and drawing the full 75W x 2 from the motherboard, and also separately drawing 300W (150W x 2) directly from the power supply.

    Your fans will generally use the 150W 8-pin connector (and that will draw directly off of your power supply) and the GPU itself will normally use the "clean power" coming off the 6-pin connector (and you can draw that directly from the motherboard)."
  8. Synchro3 macrumors 68000


    Jan 12, 2014
    Use an external power supply, or this solution:
  9. ActionableMango macrumors G3


    Sep 21, 2010
    IMHO, this is a lot of awful lot of workarounds and compromises just to have a card that's hardly any faster in the first place.
  10. Troy2000, Jul 20, 2015
    Last edited: Jul 20, 2015

    Troy2000 macrumors regular


    Oct 21, 2009
    To answer your original question, yes, you can absolutely do that. What you will need to do is purchase two 6-pin splitters and connect them such that each of the two original PCI-E power cables is supplying a 6-pin input on each card.

    This approach is generally frowned upon, however in the specific scenario which you have outlined it is perfectly safe to do. For the purposes of simplicity, let's assign each of these GPUs an arbitrary TDP of 150W and an idle power consumption of 15W.

    Snow Leopard lacks driver support for the 7950, which means that while the 5870 may consume 150W under load, the 7950 will never be running above idle. As a result, total GPU power consumption will never exceed 165W under Snow Leopard.

    Mavericks has native driver support for both the 7950 and 5870, which means it is possible for both GPUs to exceed idle power consumption while running. To counter this, all you will need to do is disconnect all video cables from the 5870 prior to booting Mavericks, which will prevent it from exceeding idle power consumption. As a result, total GPU power consumption will never exceed 165W under Mavericks

    Once again, all power consumption values in this post are arbitrary for the purposes of explanation.
  11. knowhate, Jul 21, 2015
    Last edited: Jul 16, 2016

Share This Page