Redhat Enterprise Linux 4: reboot after install hangs on "GRUB loading stage 2"

The problem came from a bad disk layout (sda1=3 GB, swap=1 GB). A boot partition must exist and layout becomes:
sda1=100 MB (/boot), sda2=2.9 GB (/), swap=1 GB.
No other choice than reinstalling to correct the layout.