[syslinux] Cannot chainload a formerly working Linux system

[alex lupu]

Chainloading a Linux system with syslinux fails unexpectedly with the lines:

WARN: No MBR magic, treating disk as raw.
ERR: Unable to find requested disk / partition combination.
boot:

coming after the usual lines
 SyslinuxBOOT ....                       (in UEFI menu)
 linuxBLFS                               (in syslinux menu)

SUMMARY (additional gory details available upon request)
~~~~~~~~~
Latest ASUS motherboard, Z270I (UEFI 2.6)
SSD with Arch on it. AOK.  The "staging" point I use for tests on an
 SSD with "old" Linux system (BLFS) to be booted with syslinux.
SSD (M.2) with Win10. AOK. NOT IN THE PICTURE (afaic).

DETAILS (a few I consider basic)
~~~~~~~
- Arch SSD:  latest version, kern 4.12.12.  64 bits.  /dev/sda.
- BLFS SSD:  left "frozen" (in good condition) at kern 4.5.  32 bits.
/dev/sdb.
- syslinux 6.3, installed on /dev/sdb first partition.  Works OK (as far as
it  goes).

Device     Boot     Start       End   Sectors   Size Id Type
/dev/sdb1  *         2048   2042239   2040192 996.2M ef EFI (FAT-12/16/32)
/dev/sdb2         2042240   3070399   1028160   502M 82 Linux swap / Solaris
/dev/sdb3         3072000 244244479 241172480   115G 83 Linux
/dev/sdb4       244244480 499196591 254952112 121.6G  7 HPFS/NTFS/exFAT

The BLFS system (on sdb3) used to be booted from a Win7 (sdb4), no longer
active now
(hence the current attempts at booting through syslinux).
First sector of sdb3 (with "legacy", 0.97, GRUB STAGE2 used to be copied to
Windows,
creating a dual boot with Windows through a menu set-up by Windows BCD
Editor.

File 'EFI/syslinux.cfg':
DEFAULT menu.c32
SAY Now booting the kernel from SYSLINUX...
LABEL linuxBLFS
   COM32 chain.c32
   APPEND hdX 3
#KERNEL LFSkernel-4.4.5-1
#APPEND root=/dev/sdb3 video=HDMI-A-2:640x480M

Notes on file 'syslinux.cfg' above:
1. X=0-5.  I also tried (in desperation) higher values for X (to no avail).
2. I chose 0-5 based on the MAP/fs results from UEFI Shell v2.2 run from
/dev/sda1.
3. For reference, FS0 -> sda1.  FS1 -> sdb1.
4. I tried other values for partition number (also in desperation).
5. The last two lines (LFSkernel and root) are temporarily commented out for
   readability.  syslinux would obviously never have a chance to run them.

COMMENTS
~~~~~~~~~~
1.  The first WARN message is particularly mystifying.
The MBR of /dev/sdb may have suffered a lot of abuse over the years but a
byte
examination of it still shows a perfect 4-partition table and the x55AA at
the end.
(no surprise there;  all Arch linux utilities like fdisk -l, hexdump, dd,
efibootmgr,
etc. work just fine and I can still access any of my files left (in OK
condition,
btw) on /dev/sdb3.  So, it seems, the MBR has managed to clearly retain its
magic.

2.  Per above, the ERR is also puzzling.  Unless syslinux identifies drives
and
partitions differently than Linux utilities and/or UEFI Shell.

Any help on clearing up / troubleshooting the situation will be highly
appreciated.
Thanks,
-- Alex