The micro SD card in Jellybean is a lot harder to find compared to ICS. I wanted to test my shiny new Sandisk class 10 64GB micro SD card to see if I was actually getting class 10 performance for writing. SdTools didn't default to the external SD card like it did for me in ICS. In the SdTools menu, I used the "custom path" option to try to get to storage/extSdCard, but there was nothing listed under storage. I knew it was there from using ES File Explorer (it will put the external SD card in your "favorites" automatically).
I wound up finding the "advanced path" option in the SDTools menu that lets you type out a path. Using /storage/extSdCard for that path lets SDTools see the micro SD card and run the test.

Reading could be better, but the write speed is as advertised.