Clarification, for beerstein:
BIOS search for first HDD (my BIOS setup HDD, PXE etc) and give control to it.
On first HDD, the MBR code (first 440 biti in /dev/sda, out of the file system, is grub stage 1) scan partition table for an ACTIVE boot partition.
my /dev/sda1 is the bootable one; here is grub1 stage 2 installed on an ext2 file system. All is OK here.
extlinux is installed on /dev/sda3 (primary partition). grub CAN give control to extlinux ( by chain loading). All is OK here also.
The problem is when extlinux is installed on /dev/sda13 (a LOGICAL partition). Because extlinux installs a file in mounted partition, at /dev/sda13/boot/extlinux/ldlinux.sys;
AND also a file (hidden, because is out of the ext2 file system) something like /dev/sda13/ldlinux.bin; this second file is like the MBR code but in logical partition. this hidden ldlinux.bin use ldlinux.sys to read the ext2 file format etc...
The error message "Boot error" came from altmbr.bin in the start of the logical partition.
I think my problem is altmbr.bin need a manually made "partition table" with LBA (logical block address) in it, to jump to start the ldlinux.bin
( the purpose to have extlinux is for its skills to dynamically concatenate initrd, so quick remaster on the fly, by using APPEND=path/to/core1.gz, /path/to/mycore2.gz; grub1 can not do it, it need first something like
cat core1.gz, mycore2.gz > mycore3.gz, not so easy to split again...)