I had the same thing happen. One if my units has a 4gb sd card and works fine. Went to get one for my other unit and smallest could find was 16gb. It will not work with it so its running without one. I'd pay the little extra to have the roku come with one installed, if finding one yourself is hit or miss. Try a smaller capacity is my advice.
This worked well for me to format using the Roku App on my smartphone, but the remote still didn't work, so I found out you have to open the battery opening in the back, press the "remote pairing button" until the light blinks green (or in newer models, it's on the back), it will re-pair your remote to your Roku. Also, once the micro SD card is in and formatted, you need to click on the app to "load" it to the micro SD card, then you won't have to reload the channel again.