I have a copy of the Slackware 64 bit dvd installation iso. I've burned this image to a dvd and used it to properly install the distro. I wanted to try also having this image available on a usb flash drive. I divided a 32 GB drive into two partitions: 1, about 22 GB for data; and 2, about 10 GB for messing around with installers (set as bootable). I dd'd the contents of the installation iso to the bootable partition on the flash drive. However, when I tried booting from the usb installer, I was greeted with a message about not being able to find an operating system.

I've previously installed Slackware to a bootable partition on a flash drive and had it properly boot on different machines. This time around, I'd like to have the installer rather than an installed system though.

What am I doing wrong?

You can use Eric's script to create an installation USB stick. It is in the "usb-and-pxe-installers" directory. The script is called: "usbimg2disk.sh".

I think the first (obvious) answer is that your /etc/fstab entries for / and /boot probably point to /dev/sda (your first hard disk). Chances are then that the machine you booting the USB from already has a disk in it and the USB is now /dev/sdb (or higher). But there are also timing issues to do with when USB modules are loaded. What you are wanting to do is definitely possible - just not as simple as what you expect it to be. But if you copied (via) dd to another hard drive, and made that the first disk on a machine, chances are good that it will boot.

I think this has to do with the difference in filesystems, and the resulting difference in isolinux vs syslinux to boot, the dvd image has to swap out isolinux for syslinux before you write it out to a usb

Try using isohybrid on the image, as in:

Fill in the name as appropriate. Then use dd from the iso image directly to the drive, not a partition, like so:

Then use a partitioning tool like cfdisk to create additional partitions if needed on the drive.