Every once in a while, a blip on the radar comes up, letting us know that there’s something not quite right in our little corner of the Mac Universe. This time around, it came in the form of Tweets directed to us, as well as in posting in popular Mac forums.

As it turns out, several other memory vendors—along with some Late 08 MacBook/MacBook Pro users—have laid claim to these machines working with 8GB of RAM. This was contradictory to our testing in December 2009 which quite clearly showed that while these models recognized a full 8GB, if an application addressed more than 6GB, the system would slow down significantly.

Not being the type to just let these sorts of claims to go unchallenged, we went back to our testing lab, grabbed the affected model machines, running 10.6.6. and dropped an 8GB upgrade kit in them.

Related: See All Upgrades Compatible with Your Mac with My Upgrades Tool

The results were exactly the same as in 2009, lending credence to our conclusions, but the sheer number of claims to the contrary led us to continue searching—and the trail ended at Apple.

In late 2009, an EFI Firmware Update was released to address the buzzing noises coming from the optical drive. However, it seems that somewhere along the line, Apple changed this update without notating it anywhere.

Whether any other elements were affected by this change is undetermined, but it did change memory addressing; with the later version of the update installed, you could address a full 8GB in Snow Leopard. Unfortunately, Software Update doesn’t show this version as being different from the previous one, so users wouldn’t be notified if they had already updated their firmware to the older version.

The practical upshot of all this is that if you installed the EFI Update when it first came out, like we did, you would have gotten the old code, which meant your computer would only address 6GB properly. Those who didn’t upgrade until after Apple changed the updater got the newer firmware, which allowed proper addressing of 8GB.

Once we manually installed the “updated” version of the EFI Firmware in our test machines, they were able to address 8GB normally, without any crashing or slowdowns.

How do you ensure your Late ‘08 MacBook/MacBook Pro can address 8GB of RAM?

First, you want to make sure you have one of the affected models:

MacBook 13.3″ 2.0GHz and 2.4GHz

MacBook Pro 15″ 2.4GHz model w/ExpressCard Slot

MacBook Pro 15″ 2.53GHz model w/ExpressCard Slot

MacBook Pro 15″ 2.66GHz model w/ExpressCard Slot

MacBook Pro 15″ 2.8GHz model w/ExpressCard Slot

MacBook Pro 15″ 2.93GHz model w/ExpressCard Slot

Next, check the Boot ROM Version in your System Profiler.

MacBook Pros with a Model ID of MacBookPro5,1 should have a Boot ROM Version of MBP51.007E.B05.

should have a Boot ROM Version of MacBooks with a Model ID of MacBook5,1 should have a Boot ROM version of MB51.007D.B03

should have a Boot ROM version of Machines with other Model IDs are not affected and don’t need an update.

**IMPORTANT NOTE: EFI Firmware Update 2.8 was released on February 28, 2012 for the Later 2008 MacBook Pro. If you have installed that update, your Boot ROM version will be MBP51.007E.B06. If you have this update installed, you’re already able to install the 8GB without any problems and don’t need to perform further updates to install more RAM.

If your Boot ROM version does not match the numbers above, download the appropriate firmware updater for your model machine and install according to the instructions:

Once you have ensured that the Boot ROM is the correct version, make sure your Operating System is updated to OSX 10.6.6. You must be running Snow Leopard or later to address 8GB on these systems, and our testing was done with 10.6.6, which is the earliest version we’re currently supporting for this.

Pardon us for our initial skepticism on this larger memory capacity finding. We had some pretty good reasons for it. First, this is the first time in our recollection that an EFI update and a dot version OS update both combined to affect memory addressing. Secondly, this reminded us of when after we were the first to qualify 2007-2009 Core 2 Duo MacBook Pro and MacBook models supporting 6GB max without experiencing a dramatic system slowdown, certain memory resellers pushed the same idea about 8GB compatibility.

Or in other shorter words, if we can’t prove either benchmark performance gains and/or system stability, we’re just not going to market a memory upgrade just to pad our MaxRAM credentials.

Your trust in us is more important than any other objective here.

UPDATE – FURTHER CLARIFICATION

Judging by the sheer number of comments, there seems to be some confusion as to which MacBooks and MacBook Pros this update applies to. We’ve created a simple flowchart that should help clear up some of the confusion.

UPDATE: FEBRUARY 2012

EFI Firmware Update 2.8 was released on February 28, 2012 for the MacBook Pro, to resolve graphics issues. It does not disable the 8GB compatibility the previous firmware revision granted.

Related: Now is the time to upgrade your memory with OWC!