Thank Curley, but i have solved on my own
isn't installer syslog supposed to record my choice during installation?
i let bios boot 2nd disk, which also has deb12, run update-grub
now it can boot newly-installed deb12 on 1st disk,
then follow stanza at
https://www.gnu.org/software/grub/manual/grub/html_node/Chain_002dloading.html#Chain_002dloading
and create my own and insert to grub.cfg
menuentry "bsd"{
insmod chain
set root=(hd0,1)
chainloader +1}
run grub-install, it's done
deb12 installer is buggy, perhaps it is confused by 3 disks(2 sata disks
and 1 usb installation stick) ?
old wisdom of dual boot still works
