I bought two SanDisk 8GB/Class 4 MicroSD Cards - one each for my two Roku 3 devices. These were the smallest and oldest cards I could seem to get from Amazon and appeared to match the requirements listed by Roku (2GB or more and Class 2 or higher).
In any case, I installed the first MicroSD Card in my first Roku 3 (label down, copper contacts up, and pushed in until it clicked). And, I was able to get the "MicroSD card detected message", after which I had my Roku 3 device format the card. I even confirmed that I no longer was getting the "channel loading ... you might want to put in a MicroSD card" message. The problems start from here...
I tried the same process with my second Roku 3 device and couldn't get it to recognize the MicroSD Card. To troubleshoot, I moved the "good card" from my first Roku 3 device to my second one - to see if the problem was with the second MicroSD Card. However, I wasn't able to get the "MicroSD card detected message": I had assumed that, even though this "good card" had already been formatted in my first Roku 3 device, the second Roku 3 device would need to reformat it.
So, I moved the "good card" back to my first Roku 3 device and, now, the first Roku 3 device doesn't recognize it (no "MicroSD card detected message" and some channels are showing the "channel loading ... you might want to put in a MicroSD card" message again).
I'm back at square 1. Any ideas? I hesitate to buy 2GB/Class 2 MicroSD Cards from Roku, given the current behavior - as I don't want to throw "good money after bad", if the root cause of the problem is something other than the size or class of the MicroSD cards I'm using.
I am having the same problem with the roku Ultra. I reformatted a new card yesterday and today, it says, microSD card detected do you need to reformat it? It does not make any sense. I am thinking the OS in the ROKU is the problem.
Known issue with San Disk SD Cards not working with Roku. There is an archived thread on reddit with many people who had the same experience. I tried two SD cards, class four 8gb San Disks and they wouldn't work. I tried reinserting, reformatting on computer, then reinserting some more. Roku never detected the card after as many as two dozen insertions. The first time I used an off brand class four Qumox 8gb card, it worked first try. Wish I'd googled issues first. Not impressed with the new channel load times though. Still takes 15 to 20 seconds to load the channel.