Take this FWIW. I purchased a Class 4 - 4GB MicroSD for this purpose as others recommended 8GB or less. Couldn't get my Roku 3 to recognize the card. I tried formatting it FAT, exFAT, FAT32 and even deleted the volume on the card. Nothing worked, until I created a full 4GB volume and formatted it with FAT32 using the default allocation cluster size and no volume name (I did this using Windows 10). The Roku recognized it and prompted me to format it. Reformatted it and it's working like a charm. I know I had attempted these settings previously, but had specified a volume name. I don't really know if specifying no volume name did the trick or if it was just reseating the card for the 12th time?
I HAD TO COME BACK AND FIND THIS BRILLIANT REMEDY TO THIS FORMATTING
MIFF, FORGOT WHAT SKEEBALL RECOMMENDED, BUT IT WORKED ON MY ROKU3.
"default allocation cluster size and no volume name"