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.