I'm attempting to use my 4 GB USB flash drive as a Slackware install environment with three partitions. A FAT 16 partition for syslinux, kernels, and initrd (just the usbboot.img provided with slackware-13.0); an ext2 partition for the Slackware packages; and a FAT 32 partition for general storage. Unfortunately I've discovered through trial and error that syslinux doesn't seem to like booting from a drive that's been partitioned (even if it's installed on the first partition). Every time I try booting with the drive partitioned I get "Error loading operating system" (different from my computer's "no os found" error, maybe indicating that it does at least try to boot syslinux).

Should I just ditch the multiple partition idea and dump everything I need into one image (and dd that onto my flash drive)? Does anyone know of a way to make this crazy plan work (lilo or grub perhaps)?

It's more that linux (and windows for that matter) doesn't deal very well with partitioned external drives, it doesn't expect something coming through the USB bus to be partitioned. In order to do what you are trying to do, you would need to define an iSCSI type attached storage device, which an external HDD could possibly be, but booting from that might be a problem, I really don't know. What I do know is I have done the image way for ubuntu a few times, with a 'pagefile' defined for persistent data. It works well. Take a look at this: https://fedorahosted.org/liveusb-creator/

I've never had a problem with any USB doing what you are doing

Are you using slacks default 3.53 syslinux?
If so you need to upgrade it

and you should make it fat32
that may also be an issue...

another thing is on one of my PC's
if I install syslinux manually, ie "syslinux -sf /dev/sdxx"
it wont boot it

But, if i use Unetbootin to put an iso on usb
it boot everytime?!

I suspect you may be using iold version of syslinux or the fat16 is issue

syslinux 3.52 actually. This version, at least, won't install ldlinux.sys to a FAT 32 partition. Will a more recent version allow me to do that?

Thanks for the tips. I'll give it a shot with a newer syslinux and report back.