Hi Orby,
infact grub4dos (used by default in tazinst) not work with ext4 filesystem.
For / (at least /boot) need ext3 fs :/.
Tazinst need adjust to use grub2 in case of target as ext4.
For workaround you can use this after install SliTaz and no reboot :
Install grub2 package
tazpkg -gi grub2
Mount target and install grub2 inside
mount /dev/sdb1 /mnt
grub-install /dev/sdb --root-directory=/mnt --no-floppy
tazpkg -gi grub2 --root=/mnt
Prepare target for chroot and configure grub2
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
chroot /mnt
Inside chroot run grub-mkconfig
grub-mkconfig -o /boot/grub/grub.cfg
exit
umount /mnt/dev /mnt/proc /mnt/sys
umount /mnt
sync && reboot
Enjoy