@bozothegeek merci, voilà le résultat de la commande mount (pour info j'avais testé les secteurs de la carte sd avant de flasher recalbox dessus avec f3):
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
none on /dev type devtmpfs (rw,relatime,size=635372k,nr_inodes=55543,mode=755)
/dev/mmcblk0p1 on /boot type vfat (ro,relatime,fmask=0022,dmask=0022,codepage=437,iocha**et=ascii,shortname=mixed,errors=remount-ro)
/dev/mmcblk0p3 on /overlay type ext4 (rw,relatime)
/dev/loop0 on /overlay/lower type squashfs (ro,relatime)
overlay on / type overlay (ro,relatime,lowerdir=/overlay_root/lower,upperdir=/overlay_root/upper,workdir=/overlay_root/work)
devpts on /dev/pts type devpts (rw,relatime,gid=5,mode=620,ptmxmode=000)
tmpfs on /dev/shm type tmpfs (rw,relatime,mode=777)
tmpfs on /tmp type tmpfs (rw,relatime)
tmpfs on /var type tmpfs (rw,relatime)
tmpfs on /run type tmpfs (rw,relatime)
/dev/sda1 on /var/recalboxfs type fuseblk (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other,blksize=4096)
/dev/sda1 on /recalbox/share type fuseblk (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other,blksize=4096)
/dev/sda1 on /media/usb0 type fuseblk (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other,blksize=4096)
overlay on /recalbox/share/system/.config/retroarch/assets type overlay (ro,relatime,lowerdir=/overlay_root/lower,upperdir=/overlay_root/upper,workdir=/overlay_root/work)
/dev/mmcblk0p3 on /recalbox/share/bootvideos type ext4 (rw,relatime)
/dev/sda1 on /run/udev/rules.d type fuseblk (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other,blksize=4096)