In the note, Apple said that this is because some RAM modules do not conform to Apple's published specifications for RAM.
Apple said: "Apple is strictly enforcing adherence to published RAM specifications. RAM that falls outside of the specification can cause stability issues with Apple hardware and software, including Mac OS X.
"If out-of-spec RAM is installed in a Macintosh computer, the system cannot be expected to perform reliably. Apple has provided RAM specifications to memory developers for some time."
Independent developer Glenn Anderson last week released a utility to repair some of these RAM modules. He claimed that one of the factors affecting Apple system's recognition of RAM pertained to the speed of the installed RAM. Some third-party RAM modules sit outside the speed specifications required by Apple.
Apple said: "The Apple 4.1.7 and 4.1.8 firmware updates incorporate a number of fixes that dramatically improve system stability and performance." Apple recommends that customers encountering disabled RAM should contact their memory vendor.