You may need a special bootdisk or dirver for SATA. It is fairly new technology:
sata.i: Serial ATA support bootdisk for Slackware 9.0.
This bootdisk is based on linux-2.4.21-pre7 with support for
Silicon Image SII3112 SATA controllers. After installing with
this bootdisk, you will probably need to download the kernel
source from ftp.kernel.org and compile new kernel modules (or
build the drivers you need into a new kernel).
There is a thread on this topic already:
It seems Slackware is not up to speed on SATA...