I think Slack uses lilo boot loader by default, which may not support booting from USB drives, might have to do some research on that. If it has lilo, you should be able to boot the computer with the Slack disc in the drive and type: linux rescue at the boot prompt and possibly get a menu with the option to re-install the boot loader, of course you want it installed in /dev/sda. Because of the error message, there may be incorrect entries or missing entries in /etc/lilo.conf. So if re-installing does not do it, have a look at this file from a CLI by typing: 'less /etc/lilo.conf' with the terminal window maximized. You should see something similar to this lilo.conf from my Mandrake:
append="acpi=ht resume=/dev/hdb2 splash=silent"
append="failsafe acpi=ht resume=/dev/hdb2 devfs=nomount"
In all these that have "root=/dev/hdb1" yours should be "root=/dev/sda2". If nothing seems wrong and the boot loader is installed in /dev/sda, then maybe you might have the option somehow to install grub boot loader instead of lilo, my Mandrake gives me the option to switch to grub in the final configuration stage of the installation process.