Ever since I reviewed the "microSD to Memory Stick Duo Adapter", I wondered whether it is simply a pass-through adapter. Recently, I learned that it is not a simple adapter, but, rather, there must be some circuitry inside.
In Akihabara of Tokyo, Japan, I saw a microSDHC to Memory Stick PRo Duo Converter MSDX2-ADP3. It sold for 980 yen, compared to the 330 yen non-SDHC adapter next to it. I was quite skeptical, but my curiosity got the better of me. I ended up buying the microSDHC to Memory Stick PRo Duo Converter and a 4 GB SDHC microSD Card.
Back at the guest house in the suburb of Osaka, Japan, I tried the microSDHC card in the regular microSD to Memory Stick Duo Adapter. But that combination did not work with the Sony Handycam DCR-HC90. Intrigued, I used the 4 GB SDHC microSD Card with the microSDHC to Memory Stick PRo Duo Converter in the camcorder. And it works. That observation indicates that the adapters are doing some conversion magic between the SD format and the Memory Stick format.
Don't try to use the microSDHC card in the regular microSD to Memory Stick Duo Adapter. The higher capacity SDHC format will damage the regular adapter. After this exercise, my regular adapter no longer works.
Later, I taken apart the regular microSD to Memory Stick Duo Adapter to see what's inside it that does the magic conversion. I found a circuit board inside with a chip on board. You can see a photo of it in the "Inside the microSD to Memory Stick Duo Adapter" thread.