Hello,
I am on the mobile so my reply will be short. I am sorry for that.
You should take a look at the Files set of instruction :
http://www.b4x.com/android/help/files.html#file
Your USB or SD card is under File.DirRootExternal. To create the necessary file's path, take a look at the File.Combine method.
And of course, you could check if the SD card is readable using File.ExternalReadable
Hope this would help you to find the right way