I have a single hard drive with three partitions: hda1 is formatted FAT16 and has a FreeDOS installation; hda2 is formatted ext3 and has a Puppy Linux 5.1.1 installation; hda3 is linux swap. Setting boot flag for hda1 gets a successful DOS boot; setting boot flag for hda2 gets a successful Linux boot. Syslinux 4.02 is on the DOS partition and I run "syslinux --install c:" then reboot to DOS (or same result with the "--stupid" option added) and get to the "boot:" prompt where I type "/linux/vmlinuz". The result is "Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,12)". I had a similar result with loadlin and was told loadlin was obsolete and I should try syslinux. Anyone have a suggestion for a lost newbie?

any reason your using syslinux on a HDD?
why not use grub-legacy or grub2?
or even grub4dos

Thanks for the grub4dos suggestion. However, running grub from the DOS command prompt to try to load and boot to vmlinuz produces exactly the same result as loadlin and syslinux: "Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,12)"

I started from scratch by reformatting and repartioning the disk while it was connected to an IDE cable instead of a USB external drive. Installed Linux Puppy 5.1.1 from the LiveCD to an ext3 partition and allowed the installation to put grub in the MBR. Then loaded FreeDOS to a FAT16 partition. Now I get the grub menu at boot and can choose either OS. I think my problem was in some boot-related configuration file that I don't understand, but will perhaps discover now that I have a functioning installation.