Ok, I googled a bit and installed PuTTY on Windows. I enabled develper mode and remote connection and I connected successfully as "nemo".

Entering "mount" brings a lot of stuff... Perhaps the last line could be interesting:
/dev/fuse on /opt/alien/storage/sdcard0 type fuse (rw,nosuid,nodev,relatime,user_id=100000,group_id= 100000,default_permissions,allow_other)

Entering "systemctl" brings again a lot of stuff. The expression "sd-mount@mmcblk1p1.service" is not in it, so I assume the service is not running.

Tomorrow I will figure out how to "mount something as root". Now I'm going to bed. Thanks for the help!