If you do decide to undertake a test on a series of Japanese SD cards, do please post your findings of BOTH those that work, along with those found to be unsuitable -
I know that my cell phone (Palm Treo) recognizes only the SD format, and not the SDHC format. So I'm limited to up to the 4GB SD size/format on my phone (I use a Transcend 150X 4GB SD (non-HC)). Pulling that SD card from my phone, it works in my Roku radio just fine. Wouldn't surprise me if our Roku radios have the same limitation (non-HC) as my phone.