I have my original Nokia 16MB MMC which is readable by my Mac (OS X) but the the new 64MB San Disk is not. Does the fact that I upgraded my firmware when I installed the larger MMC have anything to do with this problem? By the way, both cards are readable by my 9210.
What makes an MMC unreadable?
Well, you can try to format that new 64Mb MMC and see if it works on your PC.