The problem is not the amount of memory. It is the card itself. The Sandisk microSDHC card with adapter 16 GB UPC 6 19659 06688 8 will work on the Roku Ultra, but not on the Roku 3, so they may not work on the Roku 2 either.
Purchased a Samsung EVO 16 GB Class 10 microSDHD UHS-I card with adapter UPC 8 87276 96937 4 from Amazon. It works just fine.
I have relied on San Disk for years for my camera, but their cards, regardless of the amount of memory, just didn't work on the older Roku models.
My Roku kept reloading channels and telling me to buy an SD card.
Put in a Sandisk Extreme MicroSDHC UHS-1 32GB card. Found out the Roku has to be booted up before it will recognize the SD card. Anyway, it formatted and used 5% or 1.7gb of 31.4, so apparently there must only be 1.5GB of storage for channels in the Ultra 4670X