Thanks for the feedback and welcome to the Netduino community!
Have you upgraded to the v18.104.22.168 firmware? It should throw an exception if the card is not SPI compatible (instead of requiring that the card be removed).
Also, if you format the card on your PC (with FAT16 or FAT32) and re-insert it in your Netduino, does it work alright?
Finally, do you have a PN for the MicroSD card? We've used Sandisk cards that work great, so this is interesting...
I had upgraded the firmware. Your suggestion of formatting the card seems to have fixed everything. I had verified that it was formatted as FAT before using it so I'm not sure what's different, but I'm sure glad to be able to start working with the Netduino Plus now. Thanks!