Just went throught his myself. Have two Roku 3s, and one died recently from lack of use (really! maybe five hours total on it). So replaced it with an LT (2017?), and added another LT to a third TV.
After doing some reading, and from experience with picky dashcams, ordered Transcend 4 GB C10 cards. The LTs took them without problem; no preformatting required. But the Roku 3 (4200X?) wouldn't, despite trying everything in this thread.
Found an old 2 GB card, from an old digital camera, and reformatted it on my W10 PC. And the Roku 3 took it (including asking to reformat) without problem.
So ... Transcend Premium 4 GB C10 cards are apparently on the 3's verboten list.