Hi again,

i solved my probleme, i was coming from my swap partition.
As explained here, recalbox expect to have 3 partitions : / in the middle, /boot first, and /share last (). So with my swap partition, boot was not mounted and recalbox-boot.conf not available of course.

So here is my new setup

sda (ssd) sda1 500M BOOT sda2 5G RECALBOX sda4 55G archlinux sdb (hd) sdb1 500G SHARE

the systemd-boot entrie for recalbox

title recalbox linux /bzImage initrd /initrd.gz options root=UUID=[UUID] rw label=RECALBOX

and my /boot/recalbox-boot.conf

sharedevice=DEV [UUID] sharewait=30

every thing is running well now, with no hack at all