Roku Customer Service came up with a solution to my Roku 3 failing to detect micro SD cards. Sam from Roku CS suggested that I "check the status by using any Class 10 Micro SD card". I inserted a 32 GB, Class 10, micro-SDHC and the Roku detected it immediately and asked whether I wished to format the card. My previous attempts to install 4 GB, Class 4, SDHC failed because the Roku didn't detect the card, therefore didn't give me the option to format it.
Note: The 32 GB, Class 10, micro SDHC that worked in my Roku 3 had previously been formatted for my Canon camera. So, I formatted the 4GB, Class 4, SDHC in my camera and tried once again to install it in my Roku. As before the formatting, the Roku failed to detect the card. So, I conclude that the formatting does not make any difference but the Class 10 does. Good luck out there.
01/22/2016 Oops... it's back to the drawing board. I purchased a new Lexar 16 GB, Class 10 micro-SDHC and my Roku 3 failed to detect it. I wonder what the Roku 3 is looking for in order to "detect" that a micro-SD has been inserted. Until this issue is resolved I am going to suspend all channel development for the Roku platform.
01/26/2016 Today I inserted a Lexar, 32 GB, Class 10, micro-SDHC. It was detected by the Roku 3 which gave me the option of formatting the memory card.
Two of the five (40%) cards that I inserted into the Roku 3 were detected. That's a 60% failure rate.
The two cards that were detected were 32 GB, Class 10, micro-SDHC. One was Lexar brand. The other was a cheap no-name brand from China.
The three cards that the Roku 3 failed to detect were two different Transcend, 4 GB, Class 4 and one Lexar, 16 GB, Class 10.