From telnet REPL I have :

os.listdir('/sd')
Traceback ......
File .....
OSError: 19
From micropython forum kfricke give me the following answer:
"The was a change regarding the used FAT library
and filesystem mount points. " Then suggested to
file an issue to githib.
The same suggestion I have had from pycom forum.
To be able to read the SD card I have than reflashed the v1.8.6 firmware
Thank