Mac Pro: Incompatible RAM modules?

Posted by Pierre Igot in: Macintosh
February 2nd, 2007 • 11:27 am

If I was a superstitious man, I would definitely think that I am jinxed—at least when it comes to my now secondary computer, which used to be a defective G5 and now is a second Mac Pro.

Regular Betalogue readers will remember, following my purchase of a new G5 Quad back in November 2005, I enjoyed several months of happy and relatively carefree computing (except for a small problem with the video card), until the computer started acting up in a pretty bad way.

After many months of fruitless troubleshooting and dealing with various Apple representatives, I finally got Apple to send me a replacement machine. That replacement machine happened to be the exact same machine that, a few months earlier, I had obtained from my employer as a replacement for my aging G4 MDD.

So I ended up with two Mac Pro computers. There were only minor differences between the two machines, i.e. the G5 replacement had the same options as the G5 Quad, i.e. AirPort/Bluetooth, whereas the G4 replacement didn’t have the wireless hardware. (I don’t really need it, since both machines are in the same room as the AirPort Base Station and can be connected to the network with Ethernet wires.)

The other major difference was that the G4 replacement had 4 x 1 GB of additional third-party RAM that I had bought for it, whereas the G5 replacement came with the default 2 x 512 MB of RAM. The 4 x 1 GB of RAM that I had bought for the G5 were obviously useless to me now, and I managed to convince Apple’s Customer Relations that they could really help alleviate the pain of having had to deal with the defective G5 by sending me a bit of extra RAM for the Mac Pro. They promised that they would send me an extra 1 GB of RAM.

But even then, they still managed to screw things up. Anyone who has looked at the RAM requirements of the Mac Pro that RAM modules can only be installed in pairs. I was expecting to receive 2 modules of 512 MB each. Instead, I actually received a single module of 1 GB of RAM.

All you can do is laugh, really. It’s either sheer incompetence or brain-dead bureaucracy that creates such situations. I got on the phone again with my contact person, and I could easily imagine her shaking her head just like I had shaken mine. In any case, she promised she would get them to send me another 1 GB module, and indeed I got it in the mail a few days later.

I installed the RAM right away (which has to go in in a specific configuration), and things were fine for a couple of days, during which I mostly used Windows XP on the Mac Pro. But then I tried to use it for, you know, Mac stuff. And that’s when it all went pear-shaped, as they say.

As soon as I tried booting with Mac OS X, I started experiencing kernel panics. It didn’t matter whether I was booting from my hard drive or from the system discs that came with the machine. The machine was having kernel panics all over the place—usually during the booting sequence itself, or soon after it had finished booting.

If I removed the 2 x 1 GB modules, the kernel panics stopped.

If I removed the 2 x 512 MB modules in riser A and replace them with the 2 x 1 GB modules, and started the Mac Pro with only these 2 GB of RAM, I would get no kernel panics. But as soon as I tried to put the 2 x 512 MB modules back, for a total of 3 GB, I would get kernel panics.

There aren’t many options for module combinations on the Mac Pro. You have to put matching modules, and they have go in a certain order. So the only two options I have are 2 x 1 GB in riser A and 2 x 512 MB in riser B or 2 x 512 MB in riser A and 2 x 1 GB in riser B.

With both combinations, I would get kernel panics. But if I only put the 2 x 512 MB in riser A or only the 2 x 1 GB in riser A, then I would get no kernel panics.

I was able to boot from the Apple Hardware Test disc with the 3 GB installed, but of course the AHT said that everything is fine with the RAM modules.

Also, sometimes, when I tried to install 2 x 1 GB in riser A and 2 x 512 MB in riser B or 2 x 512 MB in riser A and 2 x 1 GB in riser B, I was able to boot and work with no kernel panics, but the system profiler information would only show the RAM in riser A, so it was as if the RAM in riser B wasn’t there at all.

It was not a heat issue, because the kernel panics occurred as soon as I start up, even if the machine was dead cold. It was also not a seating issue, because I was always very careful to make sure that everything was fully tucked in and snapped shut.

It was all quite mysterious. Can there be incompatibility between RAM modules? All the RAM modules came from Apple, and they all looked the same, with the same oversized heatsink. The tech specs of the RAM modules indicated by System Profiler was also the same, except for the RAM capacity, of course.

Was it the RAM modules themselves, or something in the logic board?

On the other Mac Pro with the stock 2 x 512 MB that came with it and then 4 x 1 GB of third-party RAM from another vendor (with a smaller heatsink), I have never had any problems.

I also took a look at the panic logs that the machine had been able to generate during some of the kernel panics, and they all said the same thing:

Wed Jan 31 13:38:36 2007
panic(cpu 0 caller 0x0063CCD0): Uncorrectable Fbd memory error detected. ferr = 30000800 , nerr == 00000000

After doing some online research in Apple Discussions, I found that RAM problems with Mac Pro computers are indeed not unheard of—which is not all that surprising, since the type of RAM is rather new and has these heat issues that require those heatsinks—but most people were describing kernel panics during the course of a normal computing day and not kernel panics as soon as you boot the machine. And nobody mentioned anything about incompatible RAM modules.

I phoned my Apple customer service representative once more and she told me that I would have to go through the regular AppleCare service, which I did. After being kept on hold for half an hour, I finally managed to talk to someone, and to their credit they were quick to realize that there was indeed a problem. The technicians told the customer service representative that they would send me replacement modules for the extra 2 x 1 GB of RAM and that I would have to send the original modules back.

So that where we are at. I am waiting for the replacement modules, and hoping that it will indeed fix the problem.

But the lesson here is that getting your RAM directly from Apple certainly appears to be no guarantee that things will be smoother than with third-party RAM.

The third-party RAM that I got from for my other Mac Pro might not have the oversized heatsinks (its heatsinks are much slimmer) but it has given me zero problems. Whereas this RAM that comes directly from Apple…

The only benefit that I can see of getting the RAM from Apple is that they don’t wait until you’re returned the defective RAM before shipping the replacement modules. They ask for your credit card (in case you try to cheat) and then send you the replacement stuff right away. And it arrives pretty quickly usually, and then you just phone the courier and get them to ship the defective stuff back to Apple in the same box with the included label.

But of course, if the stuff is defective, it’s not like you can do anything with it while waiting for the replacement stuff anyway—although in this particular case I can use the Mac Pro with the 2 GB of RAM, as long as I don’t try to put the 2 x 512 MB of RAM back in as well.

It’s all rather frustrating. I am fortunate enough that I have another Mac Pro that I can use as my main work machine and is not giving me any trouble (so far!), but this on-going saga with my second machine is really reaching ridiculous proportions. It’s probably just bad luck, but statistically speaking, I am beginning to think that the amount of bad luck that I have had with Apple hardware in recent years is not giving a very good picture of Apple’s hardware quality standards.

Yet’s it is the accurate picture of my very real personal experience… Let’s see what happens with these replacement RAM modules!

4 Responses to “Mac Pro: Incompatible RAM modules?”

  1. ssp says:

    Oh my, Pierre!

    You really seem to attract RAM troubles. Whenever I read about your experiences I have to think that the occasional defects, flickerings or broken chargers on my computers aren’t all that bad…

  2. Pierre Igot says:

    It’s even worse, ssp: I actually attract hardware with strange defects, not “normal” ones. A normal defect here would be a RAM module that causes kernel panics regardless of what combination of RAM is used as long as it is involved, and possibly one that would report errors in the System Profile, since this is supposed to be RAM with error correction, after all.

    But no—that would be too simple. I get RAM modules that appear to work fine except when they are used in combination with other RAM modules, which also appear to be working fine by themselves.

    Argh! And considering that this is with the machine that actually replaces my defective G5 Quad, it’s becoming a bit rich.

    All I can hope for is that the replacement modules that Apple is sending me will fix the problem, and that it’s not something with the logic board itself.

  3. Fabmil11 says:

    I’m wondering how your system is running?
    I have the exact same problem with my Mac Pro.
    With memory in the bottom shelf I get kernel panics.
    I’m currently using Apple ram.
    Somedays , nothing. Other days , 2-3 a day.
    Here is my last panic log.

    Fri Mar 30 11:07:50 2007
    panic(cpu 0 caller 0x0062FCD0): Uncorrectable Fbd memory error detected. ferr = 30000800 , nerr == 00000000

    Backtrace, Format – Frame : Return Address (4 potential args on stack)
    0x108cd8 : 0x128d1f (0x3c9540 0x108cfc 0x131df4 0x0)
    0x108d18 : 0x62fcd0 (0x631e64 0x30000800 0x0 0x8100a4)
    0x108da8 : 0x62ff64 (0x3e09100 0x3e09100 0x2 0x820044)
    0x108e38 : 0x5b2712 (0x3e09100 0x0 0x3a39d00 0x0)
    0x108e68 : 0x5b259b (0x3a2ae00 0x0 0x0 0x1c)
    0x108e88 : 0x5c531c (0x1c 0x108ef8 0x8 0x5c6dd9)
    0x108eb8 : 0x5c54c6 (0x3a26b1c 0x17 0x3a2af4a 0x3a29d20)
    0x108f18 : 0x5d205d (0x39cbb68 0x0 0xab118330 0x11b)
    0x108f38 : 0x64a4fb (0x39cbb68 0x0 0x3953c80 0x0)
    0x108f68 : 0x5b87c5 (0x3a1c600 0x0 0x3a20c00 0x49)
    0x108f88 : 0x5b1c1f (0x3a02800 0x0 0x3a20c00 0x49)
    0x108fa8 : 0x3b9d21 (0x3a20780 0x0 0x3a20600 0x49)
    0x108fe8 : 0x19aa20 (0x25353e30 0x25353e30 0x19ba76 0xe0c000)
    0x25353f18 : 0x1a42f5 (0x18 0x1 0x25353f48 0x6)
    0x25353f38 : 0x19d871 (0x0 0x42d000 0x0 0x206)
    0x25353f58 : 0x135f23 (0x0 0xf0b 0x0 0x5e) Backtrace continues…
    Kernel loadable modules in backtrace (with dependencies):

    I’m curious to know if you still have problems.

  4. Pierre Igot says:

    My problem was solved by getting the logic board replaced.

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