This is kind of a continuation of previous related threads, but it appears that there is a specific memory upgrade for the "Santa Rosa", etc., models. In my searching for info, I came across a 4GB upgrade (Samsung) at OWC that is listed as being compatible with ONLY the Santa Rosa Macbooks or above. This, coupled with the fact that many site's "memory wizards" will not recommend anything for the 2.2 GHz upgrade, indicates that we may be on to something here. I don't have the technical expertise to gauge what the exact difference is, but I bet this is the problem with the Crucial (Micron) 4GB upgrade.