What is the Brand Name of the Micro SD HC MicroSDHC card you are referring to? The Nokia Support reponse tested a Sandisk... I've found you must give the brand name when asking if a given card has been tested with a Nokia Phone. There may be a problem with some brand names. I've had good luck with Sandisk.
Thanks all. I agree, adrianhuges is right here. Beware, the 64GB does not seem to work on Lumia 810 as the developer.nokia.com specs says. it support only 32GB. I think the specs on Lumia 810 seems incorrect. I bought a 32GB and tried that and it seems to work fine though. And I don't think anything is wrong with my original 64GB as it just seems to work fine with other devices and computer. Let me know if anyone has 64GB working fine with Lumia 810.
I am not sure the developer specs are accurate as T-mobile also list 64GB as max size. There is also a rumored update coming to enable LTE functionality on the phone as well again, lot listed on developer specs under data bearer.