Last week, we wrote about firmware updates for recent Macs which Apple issued shortly after the release of Mac OS X, and which were causing updated Macs to stop seeing some third-party memory modules. (See "Avoid Current Firmware Updates" in TidBITS-574.) Apple has now provided a statement to MacInTouch in which they say that the 4.1.7 and 4.1.8 firmware updates incorporate a new check that validates whether installed RAM is compatible to address random crashes and overall stability issues. The firmware update causes the Mac to ignore DIMMs that either don't meet Apple's specifications or that the update can't identify as compatible. Of course, this raises the question of why Apple initially didn't provide some warning about the possible consequences (which they've now done in the descriptions of these files on the Apple Software Downloads site), or why the firmware update itself doesn't test the DIMMs before installation.
Glenn Anderson, author of Qualcomm's Eudora Internet Mail Server, has stepped up to the challenge with DIMM First Aid (previously called DIMMCheck), a free utility you can run in Mac OS 9 to see if your DIMMs are likely to fail Apple's newly enforced specifications. If the test fails, DIMM First Aid can reprogram the offending DIMMs' Serial Presence Detect EEPROMs so they won't later be disabled by the firmware updater. DIMM First Aid is a 6K download.
So, before running the 4.1.7 or 4.1.8 firmware updates (which Apple says "dramatically improve system stability and performance"), be sure to check your Mac's memory with DIMM First Aid. If your DIMMs fail, use DIMM First Aid to fix them. If your DIMMs pass, it's almost certainly safe to update your firmware. You can download the firmware updates from Apple's Software Downloads Web site (search for "firmware update") or get them via the Software Update control panel, but note that the firmware updates on the Mac OS 9.1 CD-ROM that comes with Mac OS X are much older versions. We said in "Out of the Box: Installing Mac OS X" in TidBITS-574 that you shouldn't run these firmware updates; that's a mistake, and we'd encourage anyone installing Mac OS X to run at least the older firmware updates on the Mac OS 9.1 CD-ROM beforehand.
If you're in the nerve-wracking position of having the firmware updates already disable one or more DIMMs, first run DIMM First Aid, which should be able to fix the problem even after the DIMMs have been disabled. If, for some reason, that doesn't work, contact the vendor from whom you purchased the RAM. Most of them, according to a Ramseeker survey, are accepting returned DIMMs. They either replace the DIMMs or reprogram the EEPROMs for you.
In the end, Glenn Anderson deserves the highest accolades for his work in creating DIMM First Aid, in contrast with Apple, whose release of an update that could disable hardware without warning was negligent in the extreme.