If you're methodical and repetitive, you can eject and re-insert the card until it works.
On my Roku 3, I had all but given up on getting the MicroSD card to work, but forum user Skeeball over on the forums gave me new hope: https://forums.roku.com/viewtopic.php?t=101208 Their issue was with a Roku 2, but the more I read that entry the more I focused on this:
or if it was just reseating the card for the 12th time?
This stuck in my head. Was this just a mechanical engineering or manufacturing flaw?
The trick would be doing this with the screen on, which meant having the HDMI cord installed at the same time as all the inserting and ejecting. Using a wire tie as a false fingernail, I started inserting and ejecting the card as rapidly as I could. After a few seconds, a message popped up on the screen and then disappeared. I looked down. I'd stopped with the card ejected. Had it worked? I tried again for about a minute. Nothing. Maybe it only works once per boot? This is how superstitions get started. I rebooted.
Once the system started and I was on the home page again. Again I rapidly started inserting and ejecting the card. The message popped up, I panicked, my finger slipped and card ejected but only a little and the wire tie got stuck in the way. The message stayed up. It was asking if I wanted to format. Barely breathing, so as not to nudge the wire tie, I clicked yes. The card formatted. If that's how it had to be, fine. I gingerly tried to place the Roku on the mantle. The wire tie was bumped and came loose. The card, no longer held in, ejected across the room. The roku informed me that my channels would have to be reloaded. Gah!
Finally, I realized I'd have to be methodical about it. I restarted the Roku. I inserted the card. I found a two-tap cadence that had me pause after each insert. tap-eject-tap-insert. I did this a few times. A message came up on the screen. The card was being mounted. I looked down. The card was in the inserted position. I placed the Roku on the shelf. I felt simultaneously beaten and victorious. This should not have been necessary. The people need to know.
Thank you to everyone who provided instructions on the various ways to format the cards and the cluster sizes to use. I can report, it matters not. My SanDisk 32 GB card (C10, U3, V30, 4K UHD), works fine, it just has to be caught by the hardware appropriately.