When you boot with the Slackware CD1, you get a prompt that says boot: there, you can specify boot parameters, like the kernel you want to use, the root partition, etc.
At that prompt, you should type: sata.i
That kernels has SATA support, which is what you need to be able to access your disk.
You can find more information on Slackware CD or
http://www.slackbook.org