After quite a lot of different trials, I found the solution. So for others to have it easier, here how I did.
Solution is quite easy :
– log in through ssh
— first as pi user, raspberry pwd
— then as root, max2play pwd
– knowing my SDXC Card was exFat, I installed the necessary debian packages : apt-get install exfat-fuse exfat-utils
– I did verify if I could list my device with lsusb. and yes it was there.
– I verified what was mounted cat /etc/fstab (boot) and cat /etc/mtab (now). and it wasn’t mounted.
– I listed the different devices on the system (I had sda sda1 sdb sdc ; I knew what was sdb, and sdc was not possible to mount xD)
– (don’t forget to create your mount directory here for me I did : mkdir /media/exfat)
– So I mounted the sda1 partition : mount -t exfat /dev/sda1 /media/exfat
– I then used the max2play interface to register this usb mountpoint as to be mounted automatically.
this article was very usefull http://www.howtogeek.com/235655/how-to-mount-and-use-an-exfat-drive-on-linux/
NB : I suggest to your dear max2play team to include fuse packages by default. ExFat SD Cards are no longer „weird“ 😉