Apple has confirmed that its recently-released set of firmware updates have been disabling some third-party memory modules.
Apple claims that the updates include a "new check that validates the compatibility" of installed memory. This was apparently developed to minimize unexpected system crashes and to make Macs more stable.
The check will also disable "out of spec" RAM modules - this means that some Apple-owners may see their third-party memory modules disabled.
In the event that customers installing the firmware update suffer from disabled RAM, Apple implies that the fault lies with the memory vendors. Customers should approach their memory vendors to arrange replacement RAM.
Apple says the problem is caused because it has strict guidelines set for memory manufacturers, and RAM that fails to meet these should not be marketed as Apple-compatible memory.