Windows Subsystem for Linux cannot yet access external USB drives or SD cards. Only internal NTFS formatted drives can be accessed as of Build 14986.
This can be confirmed by plugging in an external USB drive or SD card and typing
you will only see
c and any other NTFS formatted internal drives if they exist.
NTFS formatted SD cards don’t work, nor do USB HDD as of Build 14986.
Workaround for external drive access from Windows
Use Cygwin, which can access any drive Windows has assigned a drive letter to