How do I chain boot from grub to syslinux?

You can definitely chainload syslinux easily from grub2. The easiest and most general way would be:

Hit 'c' at the grub menu to get a command line.

set root='hd1,msdos1'
chainloader +1
boot

where 'hd1,msdos1' is a common-enough value booting off USB, but you can search most filesystems with ls, e.g., ls (hd1,msdos1) will tell you about the part, then ls (hd1,msdos1)/ will show you files. If it's a filesystem that grub normal doesn't recognize, then you may have to install an fs module with insmod. You can use grub's search command to find it. search --no-floppy --set=root

However, you likely don't need to chainload syslinux (or anything else). grub is flexible. For example, on my alpine USB stick, I often just do this:

set root='hd2,msdos1'
configfile /boot/grub/grub.cfg

Which goes right into a menu. Of course, that's with a grub cfg on it. You can also manually setup the boot from grub w/o syslinux or grub on the disk.

set root='hd2,msdos1' # or search...
linux /boot/vmlinuz
initrd /boot/initramfs

For me, I can even do something like this on my Xen'd up Alpine USB stick

multiboot2 /boot/xen.gz
module2 /boot/vmlinuz-vanilla modules=loop,squashfs,sd-mod,usb-storage
module2 /boot/initramfs-vanilla

But, that's basically what's in the /boot/grub.cfg that seems to come with Alpine now. The point is that it could point do different and varying OS, initramfs and kernels. I think you can also directly load the syslinux binary with chainloader $path_to_bin and my grub actually has a syslinux_configfile and syslinux_source commands, so this seems to work, too:

syslinux_source /boot/syslinux.bin
syslinux_configfile /boot/syslinux/syslinux.cfg

Hope one of those works for you. Learning my way around grub has been a life-saver so many times. I have saved a linux install with a corrupt config or a moved/changed HDD on a remote machine in Amsterdam from the US by hitting grub-rescue> over serial and loading modules off the existing HDD and booting into the old partition/OS in order to fix/reconfigure it.

Using YUMI on a windows system seems to be about the easiest way to do this.

http://www.pendrivelinux.com/yumi-multiboot-usb-creator

title UBUNTU 9.1 ISO Boot

find --set-root /ubuntu9.1.iso

map /ubuntu9.1.iso (hd32)

map --hook

root (hd32)

chainloader (hd32)

add this to ur menu.lst, and copy ubuntu9.1.iso to the drive on which menu.lst exists.

Update: Based on your entry, I found the article "Grub chainloads syslinux and back", which basically says "patch grub":

• Chainloading syslinux from GRUB seems not to be possible without a patch. I don't know about GRUB2. This could be useful for multi-booting (syslinux-based) partitions.

• Chainloading syslinux from GRUB4dos (on different partitions) is possible. I managed it through the mapping, explained (again) here. It solves some problems in multi-booting usb devices.

The patch page mentions a similar patch was posted to grub-devel for Grub v2; this post may be that patch. Or this message, which points to this bug report (dated Nov.2009, so it's very recent) for a Grub v2 patch.

To use one of the patches, you'd need to download the Grub source code for your preferred Grub version, check that it doesn't include the patch already, try applying the patch, and recompile and reinstall your new Grub.

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Original answer:

I'd try testing a simple Grub entry first. This is a standard Grub entry for the Memtest86+ utility on a Debian system:

title           Debian, memtest86+
root            (hd0,0)
kernel          /boot/memtest86+.bin

Just try tweaking those with your values (change the root partition or kernel path as necessary):

title           Ubuntu LiveUSB SysLinux boot
root            (hd0,0)
kernel          /syslinux/isolinux.bin

I don't think you'd need the chainloader+ command for this task.